Base station device, terminal device, transmission method, and reception method

ABSTRACT

A base station device causing no decrease in system throughput and guaranteeing the reception quality at a terminal when a DCI directed toward the terminal is mapped to a PDCCH region and an R-PDCCH region. A search space setting unit ( 103 ) sets a search space having a plurality of allocation candidates, in both regions. An allocation unit ( 106 ) allocates control information to a resource in one allocation candidate in the search space. The ratio of the number of allocation candidates in the R-PDCCH region to the number of allocation candidates in the PDCCH region in the search space for a terminal using DM-RS in the R-PDCCH region is equal to or greater than the ratio of the number of allocation candidates in the R-PDCCH region to the number of allocation candidates in the PDCCH region in the search space for a terminal using CRS in the R-PDCCH region.

TECHNICAL FIELD

The present invention relates to a base station apparatus, a terminalapparatus, a transmission method and a reception method.

BACKGROUND ART

In 3rd Generation Partnership Project Radio Access Network Long TermEvolution (3GPP-LTE (hereinafter, referred to as LTE)), OrthogonalFrequency Division Multiple Access (OFDMA) is adopted as a downlinkcommunication scheme, and Single Carrier Frequency Division MultipleAccess (SC-FDMA) is adopted as an uplink communication scheme (e.g., seeNPL-1, NPL-2, and NPL-3).

In LTE, a base station apparatus for radio communications (hereinafter,abbreviated as “base station”) performs communications by allocating aresource block (RB) in a system band to a terminal apparatus for radiocommunications (hereinafter, abbreviated as “terminal”) for every timeunit called “subframe.”

The base station also transmits allocation control information (i.e.,L1/L2 control information) for the notification of the result ofresource allocation of downlink data and uplink data to the terminal. Asthis allocation control information, Downlink Control Information (DCI)which is downlink allocation control information is transmitted. Thereare two types of DCI (which will be described later); common DCItargeting all terminals and specific DCI targeting a specific terminal(specific terminal or terminal in a specific group).

Furthermore, control information such as DCI is transmitted to aterminal using a downlink control channel such as a Physical DownlinkControl Channel (PDCCH). Here, the base station controls the resourceamount, that is, the number of OFDM symbols, used for transmission of aPDCCH in subframe units according to the number of terminals allocatedor the like. To be more specific, the resource amount used fortransmission of a PDCCH is set variably over the entire system band inthe frequency-domain, and three OFDM symbols from a leading OFDM symbolto a third OFDM symbol of one subframe in the time-domain. The basestation reports to the terminal, a Control Format Indicator (CFI) whichis information indicating the number of OFDM symbols available fortransmission of a PDCCH with the leading OFDM symbol of each subframeusing a Physical Control Format Indicator Channel (PCFICH). The terminalreceives DCI according to the CFI detected from the received PCFICH.Furthermore, the base station transmits a HARQ Indicator (HI) indicatingdelivery acknowledgment information (ACK/NACK) for uplink data to theterminal using a Physical Hybrid ARQ Indicator CHannel (PHICH) (e.g.,see NPL-1). In LTE a frequency band having a system bandwidth of up to20 MHz is supported.

Each PDCCH also occupies a resource composed of one or more consecutivecontrol channel elements (CCEs). A CCE is a minimum unit of radioresource allocated to a PDCCH. Furthermore, a CCE is made up of aplurality of consecutive resource element groups (REGs) composed ofresource elements (REs). For example, one REG is made up of four REs. Tobe more specific, a CCE is made up of a plurality of consecutive REGs(e.g., nine consecutive REGs) among REGs not allocated as radioresources for the aforementioned PCFICH and PHICH. Furthermore, the basestation may also perform interleaving processing in REG units withresources for a PDCCH for each terminal in order to randomizeinterference.

In LTE, the number of CCEs occupied by a PDCCH (the number ofconcatenated CCEs: CCE aggregation level) is selected from 1, 2, 4, and8 depending on the number of information bits of allocation controlinformation or the condition of a propagation path of a terminal. Atthis time, allocatable CCEs are predetermined for each CCE aggregationlevel (e.g., see PTL 1). For example, when the CCE aggregation level isn (e.g., n=1, 2, 4, 8), the base station can allocate to a PDCCH for theterminal, only n consecutive CCEs starting from a CCE with a CCE index(CCE number) corresponding to a multiple of n. On the other hand, sincethe terminal cannot know which CCE is allocated to a PDCCH for theterminal and what the CCE aggregation level is, the terminal has to trydecoding (blind decoding) on a PDCCH for all CCEs which may be allocatedto the PDCCH for the terminal using a round-robin method. For thisreason, as described above, it is possible to reduce the number oftrials of PDCCH decoding at the terminal by providing constraints(tree-based structure) of CCEs allocatable to the PDCCH.

Furthermore, if a base station allocates a plurality of terminals to onesubframe, the base station transmits a plurality of items of DCI via aplurality of PDCCHs at a time. In this case, in order to identify aterminal to which each PDCCH is transmitted, the base station transmitsthe PDCCH with CRC bits included therein, the bits being masked (orscrambled) with a terminal ID of the transmission destination terminal.Then, the terminal performs demasking (or descrambling) on the CRC bitsof a plurality of PDCCHs for the terminal with its own ID, therebytrial-decoding (hereinafter, referred to as “blind-decoding”) the DCI todetect the DCI for the terminal.

Also, for the purpose of reducing the number of DCI blind decodingoperations on a terminal, a method for limiting CCEs targeted for blinddecoding for each terminal is under study. This method limits a CCEregion that may be targeted for blind decoding by each terminal(hereinafter, referred to as “search space (SS)”). There are two typesof search space; common search space (hereinafter, referred to as“C-SS”) and terminal (UE) specific search space (or UE specific byC-RNTI Search Space: hereinafter, referred to as “UE-SS”). The terminalperforms blind decoding on DCI in a C-SS and DCI in a UE-SScorresponding to the terminal.

A C-SS is a search space common to all the terminals, indicating a rangeof CCEs in which all the terminals perform blind decoding on DCI. A C-SSis allocated with a PDCCH which is simultaneously reported to aplurality of terminals for transmitting control information for dataallocation common to terminals (e.g., dynamic broadcast channel (D-BCH),paging channel (PCH) and RACH response or the like) (hereinafter,referred to as “allocation control information for a common channel”). AC-SS includes six candidates targeted for blind decoding in total,namely, 4 candidates (=16 CCEs (=4 CCEs×4 candidates)) and 2 candidates(=16 CCEs (8 CCEs×2 candidates)) with respect to the CCE aggregationlevel, 4 and 8, respectively.

On the other hand, a UE-SS is a search space specific to each terminaland is randomly configured for each terminal. For example, a UE-SS ineach terminal is configured using a terminal ID of each terminal and ahash function which is a function for randomization. The number of CCEsthat forms this UE-SS is defined based on the CCE aggregation level of aPDCCH. For example, the number of CCEs forming search spaces is 6, 12,8, and 16 in association with CCE aggregation levels of PDCCHs 1, 2, 4,and 8 respectively. That is, the number of blind decoding regioncandidates is 6 (6 CCEs (=1 CCE×6 candidates)), 6 (12 CCEs (=2 CCEs×6candidates)), 2 (8 CCEs (4 CCEs×2 candidates)), and 2 (16 CCEs (=8CCEs×2 candidates)) in association with CCE aggregation levels of PDCCHs1, 2, 4, and 8 respectively. That is, blind decoding region candidatesare limited to 16 candidates in total. For example, a UE-SS is allocatedwith a PDCCH for transmitting uplink scheduling information and downlinkscheduling information directed to the target terminal.

Thus, each terminal needs only to perform blind decoding on only a groupof blind decoding region candidates in search spaces (C-SS and UE-SS)allocated to the terminal in each subframe, allowing the number of blinddecoding operations to be reduced.

Here, a C-SS and a UE-SS may be configured so as to overlap each other,or UE-SSs may also be configured so as to overlap each other. However,when UE-SSs for a plurality of terminals overlap each other, a case maybe assumed where the base station cannot allocate CCEs to a PDCCHdirected to a specific terminal. Thus, the probability that the basestation is not allowed to allocate CCEs to a PDCCH is referred to as“blocking probability.”

For example, a case will be described where 32 CCEs of CCE0 to CCE31(CCE numbers 0 to 31) are defined. In this case, the base stationsequentially allocates CCEs to a PDCCH for each terminal.

Here, suppose, for example, CCE2 to CCE9, and CCE13 to CCE19 havealready been allocated to a PDCCH. In this case, when a UE-SScorresponding to the next PDCCH (CCE aggregation level=1) is configuredof CCE4 to CCE9, the base station cannot allocate CCEs to this PDCCHbecause CCE4 to CCE9 (all CCEs in the UE-SS) are already allocated tothe other PDCCH.

Furthermore, suppose, in another example, CCE0, CCE1, CCE6 to CCE9 andCCE13 to CCE19 are already allocated to a PDCCH. In this case, when aUE-SS corresponding to the next PDCCH (CCE aggregation level-4) isconfigured of CCE0 to CCE7, the base station cannot allocate CCEs tothis PDCCH (CCE aggregation level=4). This is because the CCEaggregation level is based on a tree-based structure, and so the basestation can allocate only 4 CCEs of CCE0 to CCE3 or 4 CCEs of CCE4 toCCE7 to this PDCCH (CCE aggregation level=4). That is, CCE0 and CCE1among 4 CCEs of CCE0 to CCE3 are already allocated to the other PDCCH,and CCE6 and CCE7 among 4 CCEs of CCE4 to CCE7 are already allocated tothe other PDCCH.

Thus, when the base station fails to allocate CCEs to the PDCCH for theterminal, the base station changes the CCE aggregation level andallocates a plurality of consecutive CCEs to the PDCCH for the terminalbased on the changed CCE aggregation level.

For example, as in the case of the aforementioned example, suppose CCE2to CCE9, and CCE13 to CCE19 are already allocated. In this case, when aUE-SS corresponding to the next PDCCH (CCE aggregation level=1) is CCE4to CCE9, CCEs cannot be allocated to this PDCCH as described above. Inthis case, the base station changes the CCE aggregation level from 1 to2. This causes the UE-SS corresponding to the PDCCH (CCE aggregationlevel=2) to be changed from CCE4 to CCE9 (6 CCEs) to CCE8 to CCE19 (12CCEs). As a result, the base station can allocate CCE10 and CCE11 tothis PDCCH (CCE aggregation level=2). Even when the CCE aggregationlevel is changed in this way, if CCEs cannot yet be allocated to thePDCCH, the base station attempts transmission or the like in the nextsubframe.

Furthermore, downlink control information transmitted from the basestation is called “DCI” as described above, and contains information onresource allocated to the terminal by the base station (resourceallocation information), and modulation and channel coding scheme (MCS).The DCI has a plurality of formats. That is, examples thereof include anuplink format, downlink multiple input multiple output (MIMO)transmission format, and downlink non-consecutive band allocationformat. The terminal needs to receive both downlink allocation controlinformation (downlink-related allocation control information) and uplinkallocation control information (uplink-related allocation controlinformation).

For example, for the downlink control information (DCI), formats of aplurality of sizes are defined depending on a method for controlling atransmission antenna of a base station and a method for allocating aresource. Among the formats, a downlink allocation control informationformat for consecutive band allocation (hereinafter, simply referred toas “downlink allocation control information”) and an uplink allocationcontrol information format for consecutive band allocation (hereinafter,simply referred to as “uplink allocation control information”) have thesame size. These formats (i.e., DCI formats) include type information(for example, a one-bit flag) indicating the type of allocation controlinformation (downlink allocation control information or uplinkallocation control information). Thus, even if DCI indicating downlinkallocation control information and DCI indicating uplink allocationcontrol information have the same size, a terminal can determine whetherspecific DCI indicates downlink allocation control information or uplinkallocation control information by checking type information included inallocation control information.

For example, the DCI format in which uplink allocation controlinformation for consecutive band allocation is transmitted is referredto as “DCI format 0” (hereinafter, referred to as “DCI 0”), and the DCIformat in which downlink allocation control information for consecutiveband allocation is transmitted is referred to as “DCI format 1A”(hereinafter, referred to as “DCI 1A”). DCI 0 and DCI 1A are of the samesize and distinguishable from each other by referring to typeinformation as described above. Hereinafter, DCI 0 and DCI 1A will becollectively referred to as DCI 0/1A.

In addition to these DCI formats, there are other formats for downlink,such as DCI format used for common channel allocation (DCI format 1C:hereinafter, referred to as “DCI 1C”), DCI format used fornon-consecutive band allocation on a downlink (DCI format 1:hereinafter, referred to as “DCI 1”) and DCI format used for allocatingspatial multiplexing MIMO transmission (DCI formats 2, 2A, 2B and 2C:hereinafter, referred to as “DCI 2, 2A, 2B and 2C”). Furthermore, thereare also other DCI formats, such as DCI formats 1B and 1D (hereinafter,referred to as “DCI 1B and 1D”). Here, DCI 1, 1B, 1D, 2, 2A, 2B and 2Care formats used depending on the downlink transmission mode of theterminal (format of specific DCI). That is, DCI 1, 1B, 1D, 2, 2A, 2B and2C are formats configured for each terminal. On the other hand, DCI 0/1Ais a format independent of the transmission mode and available to aterminal in any transmission mode. That is, DCI 0/1A is a formatcommonly used for all the terminals (common DCI format). If DCI 0/1A isused, single-antenna transmission or a transmission diversity scheme isused as a default transmission mode. In the above description, specificDCI formats (DCI 2, 2A, 2B and 2C) in which spatial multiplexing MIMOtransmission corresponding to a plurality of layers may also begenerically called “DCI family 2.” Furthermore, specific DCI formats(DCI 1, 1B and 1D) corresponding to a single layer may also begenerically called “DCI family 1.” A correlation between a DCI formatand a transmission mode is defined (e.g., see Table 7-1-5 of NPL-4).

Here, DCI 1A used for common channel allocation and DCI 0/1A used forterminal-specific data allocation have the same size, and terminal IDsare used to distinguish between DCI 1A and DCI 0/1A. To be morespecific, the base station applies CRC masking to DCI 1A used for commonchannel allocation so as to be distinguishable with a terminal ID commonto all the terminals. Furthermore, the base station applies CRC maskingto DCI 0/1A used for terminal-specific data allocation so as to bedistinguishable with a terminal ID assigned in a terminal-specificmanner. Therefore, the base station can transmit DCI 0/1A used forterminal-specific data allocation also using a C-SS without increasingthe number of blind decoding operations of the terminal.

Also, the standardization of 3GPP LTE-Advanced (hereinafter, referred toas “LTE-A”), which provides a data transfer rate higher than that ofLTE, has been started. In LTE-A, a downlink transfer rate of maximum 1Gbps or higher and an uplink transfer rate of maximum 500 Mbps or higherare achieved. Therefore, a base station and a terminal capable ofcommunicating at a wideband frequency of 40 MHz or higher (hereinafter,referred to as “LTE-A terminal”) will be introduced. An LTE-A system isalso required to support a terminal designed for an LTE system(hereinafter, referred to as “LTE terminal”) in addition to an LTE-Aterminal.

In LTE-A, a transmission method using non-consecutive band allocationand a transmission method using MIMO will be introduced as new uplinktransmission methods. Accordingly, the definitions of new DCI formats(e.g., DCI formats 0A, 0B and 4: hereinafter, referred to as DCI 0A, 0Band 4)) are being studied (e.g., see NPL-4 and NPL-5). In other words,DCI 0A, 0B and 4 are DCI formats dependent on an uplink transmissionmode.

As described above, in LTE-A, if a DCI format dependent on a downlinktransmission mode (one of DCI 1, 1B, 1D, 2, 2A, 2B and 2C), a DCI formatdependent on an uplink transmission mode (one of DCI 0A, 0B and 4), anda DCI format independent of a transmission mode and common to all theterminals (DCI 0/1A) are used in a UE-SS, then the terminal performsblind decoding (monitoring) on PDCCHs of the abovementioned three DCIformats respectively. For example, since 16 blind decoding operations(blind decoding region candidates: 16 candidates in total) in one DCIformat need to be performed, 48 (=16×3) blind decoding operations intotal are performed.

Furthermore, in LTE-A, if DCI 1C and DCI 1A which are common channelallocation formats are used in a C-SS, then the terminal performs blinddecoding (monitoring) on PDCCHs of the abovementioned two DCI formats.For example, since 6 blind decoding operations (blind decoding regioncandidates: 6 candidates in total) in one DCI format need to beperformed in a C-SS, 12 (=6×2) blind decoding operations in total areperformed. Therefore, the terminal performs 60 (=48+12) blind decodingoperations in total per subframe.

Additionally, in LTE-A, to achieve an increased coverage, theintroduction of radio communication relay apparatus (hereinafter,referred to as “relay station”) has been specified. Accordingly, thestandardization of downlink control channels from base stations to relaystations (hereinafter, referred to as “R-PDCCH (Relay-Physical DownlinkControl CHannel)”) is under way (e.g., see NPLs-6, 7, 8 and 9). As aresource region to which an R-PDCCH is mapped (hereinafter, referred toas “R-PDCCH region”), a resource region to which downlink data is mapped(hereinafter, referred to as “PDSCH (Physical Downlink Shared CHannel)region”) is used.

At present, the following matters are being studied in relation to theR-PDCCH.

(1) A mapping start position in the time-domain of an R-PDCCH is fixedto a fourth OFDM symbol from a leading symbol of one subframe, and thusdoes not depend on the rate at which a PDCCH occupies OFDM symbols inthe time-domain.

(2) As a mapping method in the frequency-domain of an R-PDCCH, twodisposing methods, “localized” and “distributed” are supported.

(3) As reference signals for demodulation, Common Reference Signal (CRS)and Demodulation Reference Signal (DM-RS) are supported. The basestation notifies the relay station of which one of the reference signalsis used.

(4) Each R-PDCCH is divided into slot 0 (slot 0 or first slot) and slot1 (slot 1 or second slot) in one subframe in the time-domain.

(5) Each R-PDCCH occupies a resource configured of one or a plurality ofconsecutive Relay-Control Channel Elements (R-CCEs).

(6) A PDCCH notifying downlink resource allocation (hereinafter referredto as “DL grant”) is transmitted using slot 0 and a PDCCH notifyinguplink resource allocation (hereinafter referred to as “UL grant”) istransmitted using slot 1.

(7) When a data signal (hereinafter, referred to as “PDSCH”) isindicated by an R-PDCCH, a PDSCH is transmitted using only slot 1 orboth slot 0 and slot 1 (that is, data transmission using only slot 0 isnot possible).

CITATION LIST Patent Literature

PTL 1

-   Japanese Patent Application Laid-Open No. 2010-114780

Non-Patent Literature

NPL 1

-   3GPP TS 36.211 V9.1.0, “Physical Channels and Modulation (Release    9),” March 2010    NPL 2,-   3GPP TS 36.212 V9.2.0, “Multiplexing and channel coding (Release    9),” June 2010    NPL 3-   3GPP TS 36.213 V9.2.0, “Physical layer procedures (Release 9),” June    2010    NPL 4-   3GPP TS 36.213 V10.0.1, “Physical layer procedures (Release 10),”    December 2010    NPL 5-   3GPP TSG RAN WG1 meeting, R1-092641, “PDCCH design for Carrier    aggregation and Post Rel-8 feature,” June 2009    NPL 6-   3GPP TSG RAN WG1 meeting, R1-102700, “Backhaul Control Channel    Design in Downlink,” May 2010    NPL 7-   3GPP TSG RAN WG1 meeting, R1-102881, “R-PDCCH placement,” May 2010    NPL 8-   3GPP TSG RAN WG1 meeting, R1-103040, “R-PDCCH search space design”    May 2010    NPL 9-   3GPP TSG RAN WG1 meeting, R1-103062, “Supporting frequency diversity    and frequency selective R-PDCCH transmissions” May 2010

SUMMARY OF INVENTION Technical Problem

In the future, it is anticipated that various devices for machine tomachine (M2M) communication or the like will be introduced as radiocommunication terminals or the number of terminals applying spatialmultiplexing MIMO transmission to improve throughput will increase inresponse to an increase in the volume of content communicated, that is,the number of pieces of control information with the large amount ofinformation will increase. In consideration of these anticipations,there is concern that there may be a shortage of resources in a resourceregion to which a PDCCH is mapped (hereinafter, referred to as “PDCCHregion”). When a PDCCH cannot be mapped due to a shortage of resources,downlink data cannot be allocated to the terminal, and therefore evenwhen there is free space in the resource region to which downlink datais mapped (hereinafter, referred to as “PDSCH region”), the free spacecannot be used, and there is concern that the system throughput maydeteriorate.

Thus, LTE-A is studying the possibility of mapping DCI for a terminalconnected to a base station (the terminal under the control of the basestation) to the aforementioned R-PDCCH region in addition to the PDCCHregion. When transmitting a PDCCH for a terminal also in the R-PDCCHregion, a frequency division multiplexing (FDM) configuration (see FIG.1A) and a time division multiplexing+frequency division multiplexing(TDM+FDM) configuration (see FIG. 1B) as shown in FIG. 1 are possible.In the FDM configuration shown in FIG. 1A, signals in the R-PDCCH regionare not multiplexed in the time-domain but multiplexed only in thefrequency-domain. On the other hand, in the TDM+FDM configuration shownin FIG. 1B, the R-PDCCH region is configured of two slots (called slot 0and slot 1, or 1st slot and 2nd slot) in the time-domain. Furthermore,in the TDM+FDM configuration, signals in the R-PDCCH region aremultiplexed in both the time-domain and the frequency-domain.

However, simply adding an R-PDCCH region in addition to a PDCCH regionas a region for transmitting DCI for a terminal connected to a basestation may disadvantageously lead to an increase in the number of blinddecoding operations to be performed by the terminal, resulting inincreases in power consumption, processing delay of the terminal, andcircuit scale. For example, according to the above-describedconfiguration of a search space, in one subframe, a search space isconfigured for each of a PDCCH region, an R-PDCCH region of slot 0 andan R-PDCCH region of slot 1. Thus, if the number of blind decodingoperations to be performed by a terminal in each region is 60 asmentioned above, the terminal would repeat 180 blind decoding operations(=60×3 regions) in total for each subframe. In other words, the numberof blind decoding operations increases and the configuration of aterminal becomes complicated.

On the other hand, another possible approach of configuring the searchspace is allocation of a search space to each of a PDCCH region and anR-PDCCH region (slot 0 and slot 1) under the assumption that the totalnumber of region candidates for blind decoding to be performed by aterminal in one subframe (i.e., the total number of blind decodingoperations) is set to the number used in the related art as described(e.g., 60 operations). However, in this case, the size of a search spacein each of the PDCCH region, the R-PDCCH region of slot 0 and theR-PDCCH region of slot 1 is substantially reduced to ⅓, and thus theaforementioned blocking probability may increase. For that reason,inefficient use of resources may cause a decrease in system throughput.

Furthermore, an environment in which various modes of cell such as afemto cell and pico cell in addition to a macro cell coexist (e.g.,heterogeneous network (HetNet) environment made up of macro cell andpico cell/femto cell) is under study. However, in an environment inwhich cells in various modes coexist, there is concern that interferencein a PDCCH region of each cell may increase due to influences from othercells. When, for example, a terminal connected to a macro cell(Non-closed Subscriber Group (Non-CSG) terminal) is located in proximityof a femto cell, the Non-CSG terminal receives large interference fromthe femto cell. Alternatively, when a terminal connected to a pico cellis located near a cell edge of the pica cell (e.g., range expansionregion), the terminal receives large interference from the macro cell.For this reason, in the PDCCH region, PDCCH reception performance ofeach terminal deteriorates.

Thus, when a search space is allocated to each of a PDCCH region and anR-PDCCH region (slot 0 and slot 1), there are problems that systemthroughput and the PDCCH reception performance in each terminaldeteriorates depending on the configuration of the search space in eachregion.

It is an object of the present invention to provide a base station, aterminal, a transmission method and a reception method that can preventdeterioration of system throughput and secure desired receiving qualityin the terminal even when a PDCCH for the terminal connected to the basestation is mapped to both a PDCCH region and an R-PDCCH region.

Solution to Problem

A base station according to a first aspect of the present invention is abase station apparatus that can transmit control information using afirst control channel using a common reference signal that can bereceived by all terminal apparatuses and a second control channel usingthe common reference signal or a specific reference signal for eachterminal apparatus, and adopts a configuration including: a search spaceconfiguring section that configures a search space having a plurality ofallocation candidates in a first resource region to which the firstcontrol channel is mapped and a second resource region to which thesecond control channel is mapped; and an allocating section thatallocates the control information to a resource in one allocationcandidate from among the plurality of allocation candidates in thesearch space, in which a ratio of a number of the allocation candidatesin the second resource region to a number of the allocation candidatesin the first resource region in the search space configured in theterminal apparatus using the specific reference signal in the secondcontrol channel is equal to or greater than a ratio of a number of theallocation candidates in the second resource region to a number of theallocation candidates in the first resource region in the search spaceconfigured in the terminal apparatus using the common reference signalin the second control channel.

A terminal according to a second aspect of the present invention adoptsa configuration including: a first receiving section that blind-decodesa search space having a plurality of allocation candidates configured ina first resource region to which a first control channel using a commonreference signal that is receivable by all terminal apparatuses ismapped and a second resource region to which a second control channelusing the common reference signal or a specific reference signal foreach terminal apparatus is mapped and obtains control informationtransmitted using the first control channel or the second controlchannel; and a second receiving section that receives a downlink datasignal based on the control information for the terminal apparatusallocated to one of the plurality of allocation candidates, in which aratio of a number of the allocation candidates in the second resourceregion to a number of the allocation candidates in the first resourceregion in the search space configured in the terminal apparatus usingthe specific reference signal in the second control channel is equal toor greater than a ratio of a number of the allocation candidates in thesecond resource region to a number of the allocation candidates in thefirst resource region in the search space configured in the terminalapparatus using the common reference signal in the second controlchannel.

A transmission method according to a third aspect of the presentinvention is a transmission method performed by a base station apparatusthat transmits control information using a first control channel using acommon reference signal that is receivable by all terminal apparatusesand a second control channel using the common reference signal or aspecific reference signal for each terminal apparatus, the transmissionmethod including: configuring a search space having a plurality ofallocation candidates, in a first resource region to which the firstcontrol channel is mapped and a second resource region to which thesecond control channel is mapped; and allocating the control informationto a resource in one allocation candidate from among the plurality ofallocation candidates in the search space, in which a ratio of a numberof the allocation candidates in the second resource region to a numberof the allocation candidates in the first resource region in the searchspace configured in the terminal apparatus using the specific referencesignal in the second control channel is equal to or greater than a ratioof a number of the allocation candidates in the second resource regionto a number of the allocation candidates in the first resource region inthe search space configured in the terminal apparatus using the commonreference signal in the second control channel.

A reception method according to a fourth aspect of the present inventionis a reception method that adopts a configuration including:blind-decoding a search space having a plurality of allocationcandidates configured in a first resource region to which a firstcontrol channel using a common reference signal that is receivable byall terminal apparatuses is mapped and a second resource region to whicha second control channel using the common reference signal or a specificreference signal for each terminal apparatus is mapped and obtainingcontrol information transmitted using the first control channel or thesecond control channel; and receiving a downlink data signal based onthe control information for the terminal apparatus allocated to one ofthe plurality of allocation candidates, in which a ratio of a number ofthe allocation candidates in the second resource region to a number ofthe allocation candidates in the first resource region in the searchspace configured in the terminal apparatus using the specific referencesignal in the second control channel is equal to or greater than a ratioof a number of the allocation candidates in the second resource regionto a number of the allocation candidates in the first resource region inthe search space configured in the terminal apparatus using the commonreference signal in the second control channel.

Advantageous Effects of Invention

According to the present invention, even when a PDCCH for a terminalconnected to a base station is mapped to both a PDCCH region and anR-PDCCH region, desired receiving quality can be secured in the terminalwithout causing deterioration of system throughput.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates an example of multiplexed configuration of R-PDCCHregions;

FIG. 2 is a diagram illustrating downlink resources according toEmbodiment 1 of the present invention;

FIG. 3 is a diagram for explaining features of a reference signalaccording to Embodiment 1 of the present invention;

FIG. 4 is a principal block diagram of a base station according toEmbodiment 1 of the present invention;

FIG. 5 is a principal block diagram of a terminal according toEmbodiment 1 of the present invention;

FIG. 6 is a block diagram illustrating the configuration of the basestation according to Embodiment 1 of the present invention;

FIG. 7 is a diagram illustrating search spaces configured in theterminal according to Embodiment 1 of the present invention;

FIG. 8 is a block diagram illustrating the configuration of the terminalaccording to Embodiment 1 of the present invention;

FIG. 9A is a diagram illustrating an example of search spaceconfiguration in a PDCCH region according to Embodiment 1 of the presentinvention;

FIG. 9B is a diagram illustrating an example of search spaceconfiguration in an R-PDCCH region according to Embodiment 1 of thepresent invention;

FIG. 10 is a diagram illustrating the number of DCI allocation regioncandidates (number of blind decoding region candidates) in a PDCCHregion and an R-PDCCH region according to Embodiment 1 of the presentinvention;

FIG. 11 is a diagram illustrating an example of search spaceconfiguration according to Embodiment 2 of the present invention(configuration method 1);

FIG. 12A is a diagram illustrating an example of search spaceconfiguration in a PDCCH region according to Embodiment 3 of the presentinvention;

FIG. 12B is a diagram illustrating an example of search spaceconfiguration in an R-PDCCH region according to Embodiment 3 of thepresent invention;

FIG. 13 is a diagram illustrating search space patterns according toEmbodiment 3 of the present invention;

FIG. 14 is a block diagram illustrating an internal configuration of asearch space configuration section according to Embodiment 5 of thepresent invention;

FIG. 15 is a diagram illustrating features of a reference signalarranged in each transmission region according to Embodiment 5 of thepresent invention;

FIG. 16 is a diagram illustrating features of a downlink DCI formataccording to Embodiment 5 of the present invention;

FIG. 17A is a diagram illustrating an example of search spaceconfiguration in a PDCCH region according to Embodiment 6 of the presentinvention;

FIG. 17B is a diagram illustrating an example of search spaceconfiguration in an R-PDCCH region according to Embodiment 6 of thepresent invention;

FIG. 18 is a diagram illustrating an example of search spaceconfiguration according to a variation of Embodiment 6 of the presentinvention;

FIG. 19 is a block diagram illustrating an internal configuration of asearch space configuration section according to Embodiment 7 of thepresent invention;

FIG. 20 is a diagram illustrating an example of search spaceconfiguration according to Embodiment 7 of the present invention;

FIG. 21 is a diagram illustrating a data allocation example in eachtransmission region according to Embodiment 7 of the present invention;and

FIG. 22 is a diagram illustrating search space patterns according to thepresent invention.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will be described in detail withreference to the accompanying drawings. In the embodiments, the samereference numerals are used for denoting the same components, and aredundant description thereof is omitted.

The following matters are assumed as premises in the description below.

That is, as for a region for which DCI for a terminal connected to abase station according to the present invention is transmitted (PDCCHtransmission region), a case will be described as an example where onlya PDCCH region shown on the left side of FIG. 2 is used and where both aPDCCH region and an R-PDCCH region shown on the left side of FIG. 2 areused. Furthermore, as shown on the left side of FIG. 2, when a TDM+FDMconfiguration is applied in the R-PDCCH region, the R-PDCCH region isdivided into slot 0 and slot 1 within one subframe in the time-domain.

Furthermore, in the PDCCH region, as shown on the right side of FIG. 2,a CRS is used as a reference signal (RS) for demodulation. In contrast,in the R-PDCCH region as shown on the right side of FIG. 2, a CRS orDM-RS is used as a reference signal (RS) for demodulation. That is, inthe PDCCH region and R-PDCCH region, there are two combinations ofreference signals used for DCI demodulation by a terminal under thecontrol of the base station; (1) CRS (PDCCH region)+CRS (R-PDCCH region)and (2) CRS (PDCCH region)+DM-RS (R-PDCCH region).

Here, features of CRS and DM-RS are shown in FIG. 3. When a referencesignal to be used is a CRS, the CRS itself and control informationdemodulated using the CRS are not precoded (weighted on the transmittingside) (no precoding). That is, a CRS is an RS common to all theterminals, which is a reference signal that can be received by all theterminals. Furthermore, a CRS is disposed distributed over the entirefrequency-domain (that is, a CRS is disposed over the entirefrequency-domain), and so there is no constraint on the disposition ofcontrol information demodulated using the CRS in the frequency-domain,and therefore the control information can also be disposed distributedover the entire system bandwidth (distributed disposition) and it ispossible to obtain a frequency diversity effect with respect to thecontrol information. Therefore, use of CRS is effective in reducinginfluences of a variation in the received signal level in thefrequency-domain caused by the terminal moving at a high speed.

On the other hand, when a reference signal used is a DM-RS, the DM-RSitself and control information demodulated using the DM-RS is subjectedto precoding (weighting on the transmitting side) optimized for acertain terminal. For this reason, it is difficult for all the terminalsto receive a DM-RS while securing desired receiving quality. That is, aDM-RS is a specific RS for each terminal. Furthermore, since a DM-RS issubjected to precoding optimized for a specific terminal in thefrequency-domain, its disposition in the frequency-domain is aconcentrated disposition (localized disposition) on a certain frequencyband. This makes it possible to improve received signal power of controlinformation for a specific terminal. Therefore, use of a DM-RS iseffective for a terminal located at a cell edge or the like (terminalwhose received signal level is low).

In FIG. 3, it is assumed that a CRS is disposed distributed over theentire frequency-domain, but the CRS may be disposed only in part of thefrequency-domain (that is, localized disposition). Furthermore, thepresent invention is not limited to the use of CRS and DM-RS. That is,not only a CRS and DM-RS but also any reference signal that satisfiesone of the features of reference signals shown in FIG. 3 (presence orabsence of precoding, target terminal (common or specific) anddisposition in the frequency-domain (distributed disposition orlocalized disposition)) may be used instead of a CRS and DM-RS.

Embodiment 1

[Overview of Communication System]

A communication system according to Embodiment 1 of the presentinvention includes base station 100 and terminal 200. Base station 100is an LTE-A base station, and terminal 200 is an LTE-A terminal.

In a communication system according to the present embodiment,allocation control information is transmitted from base station 100 toterminal 200 through a PDCCH using CRSs which is a common referencesignal that can be received by all the terminals and an R-PDCCH usingCRSs or DM-RS which is a specific reference signal for each terminal.

FIG. 4 illustrates principal components of base station 100 according tothe present embodiment. In base station 100 shown in FIG. 4, searchspace configuration section 103 configures search spaces having aplurality of DCI allocation region candidates in a PDCCH region and anR-PDCCH region, and allocating section 106 allocates allocation controlinformation (that is, DCI) to resources in one DCI allocation regioncandidate among a plurality of DCI allocation region candidates in asearch space (resource composed of one or a plurality of CCEs in a PDCCHregion or resource composed of one or a plurality of R-CCEs in anR-PDCCH region).

FIG. 5 illustrates principal components of terminal 200 according to thepresent embodiment. In terminal 200 shown in FIG. 5, PDCCH receivingsection 207 blind-decodes a search space having a plurality of DCIallocation region candidates (that is, blind decoding region candidatesin terminal 200) configured in a PDCCH region to which a PDCCH usingCRSs that can be received by all the terminals is mapped and an R-PDCCHregion to which an R-PDCCH using CRSs or DM-RS for each terminal ismapped, and obtains allocation control information (that is, DCI)transmitted using the PDCCH and R-PDCCH. PDSCH receiving section 208receives downlink data signals on the basis of the allocation controlinformation for terminal 200 disposed in any one of the plurality of DCIallocation region candidates.

However, the ratio of the number of DCI allocation region candidates inthe R-PDCCH region to the number of DCI allocation region candidates inthe PDCCH region in a search space configured in terminal 200 usingDM-RSs in the R-PDCCH region is equal to or greater than the number ofDCI allocation region candidates in the R-PDCCH region to the number ofDCI allocation region candidates in the PDCCH region in a search spaceconfigured in terminal 200 using CRSs in the R-PDCCH region.

[Configuration of Base Station 100]

FIG. 6 is a block diagram illustrating the configuration of base station100 according to Embodiment 1 of the claimed invention.

In base station 100 shown in FIG. 6, configuration section 101configures a resource region for use in the transmission of DCI toterminal 200 (transmission region) and also configures each transmissionmode for uplink and downlink for terminal 200. The configuration of aresource region and a transmission mode is performed for each terminal200 to be configured.

Specifically, configuration section 101 includes transmission regionconfiguration section 131 and transmission mode configuration section132.

Transmission region configuration section 131 configures whether or notto include an R-PDCCH region in addition to a PDCCH region for eachterminal 200 as a region for use in the transmission of DCI(transmission region). For example, when there is concern that the PDCCHregion might become tight or when it is determined that interference inthe PDCCH region is large, transmission region configuration section 131configures the DCI transmission region so as to include the R-PDCCHregion in addition to the PDCCH region. Examples of the case where thereis concern that the PDCCH region might become tight include a case wheremany terminals are connected to base station 100 and a case where thereare many terminals 200 to which spatial multiplexing MIMO transmissionadaptable to a plurality of layers is allocated. That is, transmissionregion configuration section 131 determines whether blind decoding isperformed, for each terminal, on only a PDCCH region or on both a PDCCHregion and an R-PDCCH region (or only on an R-PDCCH region).

Furthermore, transmission region configuration section 131 configures areference signal for use in each transmission region. To be morespecific, transmission region configuration section 131 configures a CRSas a reference signal for use in a PDCCH region. Furthermore, when theDCI transmission region is configured to also include an R-PDCCH region,transmission region configuration section 131 configures one of a CRSand DM-RS as a reference signal for use in the R-PDCCH region. Forexample, when it is determined that terminal 200 is located at a celledge covered by base station 100 and the intensity of the receivedsignal (received signal level) needs to be improved, transmission regionconfiguration section 131 configures a DM-RS as a reference signal foruse in an R-PDCCH region to which a control signal (that is, DCI) forterminal 200 is mapped based on features of the DM-RS (see FIG. 3).Alternatively, when it is determined that terminal 200 is moving fast,transmission region configuration section 131 configures a CRS as areference signal for use in an R-PDCCH region to which a control signalfor terminal 200 is mapped based on features of the CRS (see FIG. 3).

Furthermore, transmission mode configuration section 132 configures,based on a propagation path situation for each terminal 200 or the like,the DCI format and transmission mode (for example, spatial multiplexingMIMO transmission, beam forming transmission, and non-consecutive bandallocation) of each of uplink and downlink for terminal 200.

Configuration section 101 then outputs configuration informationcontaining the information indicating the transmission region of DCI,information indicating a reference signal for use in the transmissionregion of DCI and information indicating the transmission modeconfigured in each terminal 200 to control section 102, search spaceconfiguration section 103, coding/modulation section 107 andtransmission weight configuration section 108. These items ofinformation contained in the configuration information are reported toeach terminal 200 via coding/modulation section 107 as upper layercontrol information (referred to as “RRC control information” or “RRCsignaling”). Furthermore, since the reference signal for use in a PDCCHregion is fixed to a CRS, configuration section 101 may output onlyinformation indicating a reference signal for use in an R-PDCCH region.Alternatively, when a reference signal for use in an R-PDCCH region isfixed to one of CRS and DM-RS beforehand, configuration section 101 neednot output information indicating a reference signal. These items ofinformation outputted from configuration section 101 may besimultaneously transmitted or transmitted at different timings.

Control section 102 generates allocation control information accordingto the configuration information inputted from configuration section101.

To be more specific, control section 102 generates allocation controlinformation containing MCS information corresponding to one transportblock transmitted, resource (RB) allocation information and a new dataindicator (NDI) or the like. As the resource allocation information,control section 102 generates uplink resource allocation informationindicating an uplink resource (for example, a Physical Uplink SharedChannel (PUSCH)) to which uplink data from terminal 200 is allocated, ordownlink resource allocation information indicating a downlink resource(for example, a Physical Downlink Shared Channel (PDSCH)) to whichdownlink data to terminal 200 is allocated.

Furthermore, on the basis of the configuration information received fromconfiguration section 101, control section 102 generates allocationcontrol information according to a DCI format based on a transmissionmode of the uplink from terminal 200 (i.e., one of DCI 0A, 0B and 4), aDCI format based on a transmission mode of the downlink (one of DCI 1,1B, 1D, 2, 2A, 2B and 2C) or a DCI format common to all the terminals(DCI 0/1A).

For example, during normal data transmission, control section 102generates allocation control information in a format depending on thetransmission mode for each terminal 200 (one of DCI 1, 1B, 1D, 2, 2A, 2Band 2C or one of DCI 0A, 0B and 4). As a result, data can be transmittedat the transmission mode configured for each terminal 200, whichimproves throughput.

However, an abrupt change in the condition of a propagation path or achange in interference from an adjacent cell may cause frequent errorsin receiving data in the transmission mode configured for each terminal200. In this case, control section 102 generates allocation controlinformation in the format (DCI 0/1A) common to all the terminals andtransmits data in a robust default transmission mode. As a result,robust data transmission is allowed even if a propagation environment isabruptly changed.

Also, when upper-layer control information (i.e., RRC signaling) istransmitted for the notification of a transmission mode change underdeteriorated conditions of a propagation path, control section 102generates allocation control information (i.e., DCI 0/1A) common to allthe terminals and transmits the information using the defaulttransmission mode. The number of information bits of DCI 0/1A common toall the terminals is smaller than that of DCI 1, 1B, 1D, 2, 2A, 2B, 2C,0A, 0B and 4 depending on a particular transmission mode. For thisreason, if the same number of CCEs is set, DCI 0/1A can allowtransmission at a lower coding rate than that related to DCI 1, 1B, 1D,2, 2A, 2B, 2C, 0A, 0B and 4. Thus, use of DCI 0/1A in control section102 under a deteriorated condition of a propagation path enablesterminal 200 having a poor condition of a propagation path to receiveallocation control information (and data) with a low error rate.

Control section 102 also generates allocation control information forcommon channels (for example, DCI 1C and 1A) for the allocation of datacommon to a plurality of terminals, such as broadcasting and paginginformation, in addition to the allocation control information for theallocation of terminal-specific data.

Control section 102 outputs MCS information and an NDI to PDCCHgenerating section 104, uplink resource allocation information to PDCCHgenerating section 104 and extracting section 118, and downlink resourceallocation information to PDCCH generating section 104 and multiplexingsection 110, among the generated items of allocation control informationfor the allocation of terminal-specific data. Control section 102 alsooutputs the generated allocation control information for a commonchannel to PDCCH generating section 104.

Search space configuration section 103 configures a common search space(C-SS) and specific search space (UE-SS) on the basis of a transmissionregion of DCI and a reference signal used, indicated by theconfiguration information inputted from configuration section 101. Thecommon search space (C-SS) is a search space common to all theterminals, and the specific search space (UE-SS) is a search spacespecific to each terminal as described above.

Specifically, search space configuration section 103 configures preparedCCEs (for example, CCEs from leading to 16th ones) as a C-SS. A CCE is abasic unit of the control information disposed.

Search space configuration section 103 also configures a UE-SS for eachterminal. For example, search space configuration section 103 determinesa UE-SS for a certain terminal on the basis of the ID of the terminal, aCCE number obtained by calculations using a hash function forrandomization, and the number of CCEs (L) that form a search space. AUE-SS can be configured in each of the PDCCH region and R-PDCCH region.

FIG. 7 is a diagram illustrating an example of configuration of a C-SSand a UE-SS for a certain terminal. In FIG. 7, with respect to CCEaggregation level 4 of a PDCCH, four DCI allocation region candidates(i.e., CCEs 0 to 3, CCEs 4 to 7, CCEs 8 to 11, and CCEs 12 to 15) areconfigured as a C-SS. Also, with respect to CCE aggregation level 8 ofthe PDCCH, two DCI allocation region candidates (i.e., CCEs 0 to 7 andCCEs 8 to 15) are configured as another C-SS. In other words, in FIG. 7,the six DCI allocation region candidates in total are configured as theC-SSs.

Furthermore, in FIG. 7, with respect to CCE aggregation level 1, six DCIallocation region candidates (i.e., each of CCEs 16 to 21) areconfigured as a UE-SS. With respect to CCE aggregation level 2, six DCIallocation region candidates (i.e., obtained by partitioning CCEs 6 to17 into sets of 2) are configured as another UE-SS. With respect to CCEaggregation level 4, two DCI allocation region candidates (i.e., CCEs 20to 23 and CCEs 24 to 27) are configured as yet another UE-SS. Withrespect to CCE aggregation level 8, two DCI allocation region candidates(i.e., CCEs 16 to 23 and CCEs 24 to 31) are configured as still anotherUE-SS. In other words, in FIG. 7, 16 DCI allocation region candidates intotal are configured as the UE-SSs.

Furthermore, when both a PDCCH region and an R-PDCCH region areconfigured as DCI transmission regions, search space configurationsection 103 configures search spaces (C-SS and UE-SS) having theaforementioned plurality of DCI allocation region candidates in thePDCCH region and R-PDCCH region. Details of the search spaceconfiguration processing of search space configuration section 103 willbe described later.

Search space configuration section 103 outputs search space informationindicating the configured C-SS and UE-SS of each terminal to allocatingsection 106 and coding/modulation section 107.

Returning to FIG. 6, PDCCH generating section 104 generates DCIcontaining allocation control information for the allocation ofterminal-specific data inputted from control section 102 (that is,uplink resource allocation information, downlink resource allocationinformation, MCS information and NDI or the like for each terminal, anduplink resource allocation information or downlink resource allocationinformation) or DCI containing allocation control information for acommon channel (that is, broadcast information and paging informationcommon to terminals or the like). At this time, PDCCH generating section104 adds CRC bits to the uplink allocation control information and thedownlink allocation control information generated for each terminal andmasks (or scrambles) the CRC bits with a terminal ID. PDCCH generatingsection 104 then outputs the masked DCI to coding/modulation section105.

Coding/modulation section 105 modulates the DCI received from PDCCHgenerating section 104 after coding and outputs the modulated signals toallocating section 106. Coding/modulation section 105 determines acoding rate set on the basis of channel quality indicator (CQI)information reported from each terminal so as to achieve a sufficientreception quality in each terminal. For example, as a distance between aterminal and a cell boundary decreases (i.e., as the channel quality ofa terminal deteriorates), the coding rate to be set by coding/modulationsection 105 decreases.

Allocating section 106 allocates, to each of CCEs or R-CCEs in a C-SS,or CCEs or R-CCEs in a UE-SS for each terminal, which are indicated bysearch space information inputted from search space configurationsection 103, DCI containing allocation control information for a commonchannel and DCI containing allocation control information for theallocation of terminal-specific data to each terminal, which areinputted from coding/modulation section 105. For example, allocatingsection 106 selects one DCI allocation region candidate from a group ofDCI allocation region candidates in a C-SS (for example, see FIG. 7).Allocating section 108 then allocates DCI containing allocation controlinformation for a common channel to a CCE (or an R-CCE; hereinafter,sometimes simply referred to as “CCE” without distinguishing “CCE” from“R-CCE”) in the selected DCI allocation region candidate.

In the case where a DCI format for the allocation target terminal is aDCI format dependent on a transmission mode (for example, DCI 1, 1B, 1D,2, 2A, 2B, 2C, 0A, 0B and 4), allocating section 106 allocates a CCE ina UE-SS configured for the allocation target terminal to DCI. On theother hand, in the case where a DCI format for the allocation targetterminal is a format common to all the terminals (for example, DCI0/1A), allocating section 106 allocates a CCE in a C-SS or a CCE in aUE-SS configured for the allocation target terminal to DCI.

The CCE aggregation level to be allocated to one DCI item depends on thecoding rate and the number of DCI bits (namely, the amount of allocationcontrol information). For example, more physical resources are requiredfor a coding rate set to be low of DCI for a terminal located around acell boundary. For this reason, allocating section 106 allocates moreCCEs to DCI for a terminal located around a cell boundary.

Allocating section 106 then outputs information about the CCEs allocatedto the DCI to multiplexing section 110 and ACK/NACK receiving section121. Allocating section 106 outputs the coded/modulated DCI tomultiplexing section 109.

Coding/modulation section 107 modulates the configuration informationinputted from configuration section 101 and search space informationinputted from search space configuration section 103 (that is,upper-layer control information) after channel coding and outputs themodulated configuration information and search space information tomultiplexing section 110.

Transmission weight configuration section 108 configures a transmissionweight (precoding weight) for a terminal using DM-RSs as referencesignals for demodulation based on the configuration information inputtedfrom configuration section 101 and outputs the configured transmissionweight to multiplexing section 110.

Coding/modulation section 109 modulates the input transmission data(downlink data) after channel coding and outputs the modulatedtransmission data signals to multiplexing section 110.

Multiplexing section 110 multiplexes the coded/modulated DCI signalinputted from allocating section 106, the configuration information andsearch space information (that is, upper-layer control information)inputted from coding/modulation section 107, and the data signals(namely, PDSCH signals) inputted from coding/modulation section 109 inthe time-domain and the frequency-domain.

Here, multiplexing section 110 multiplies DCI information and a PDSCHsignal or the like in an R-PDCCH region for a terminal using DM-RSs asreference signals for demodulation, by a transmission weight inputtedfrom transmission weight configuration section 108, and outputs theweighted signals to Inverse Fast Fourier Transform (IFFT) section 111for each antenna. Furthermore, multiplexing section 110 performs spatialfrequency block coding (SFBC) processing on a signal for which notransmission weight is configured (DCI information in a PDCCH region orthe like) and outputs the signal to Inverse Fast Fourier Transform(IFFT) section 111 for each antenna. Furthermore, multiplexing section110 maps the PDCCH signals and the data signals (PDSCH signals) on thebasis of the downlink resource allocation information inputted fromcontrol section 102. Multiplexing section 110 may also map theconfiguration information and search space information onto the PDSCH.

IFFT sections 110-1 and 111-2, CP (Cyclic Prefix) adding sections 112-1and 112-2 and RF transmitting sections 113-1 and 113-2 are provided forantennas 114-1 and 114-2 correspondingly.

IFFT sections 111-1 and 111-2 convert the multiplexed signals frommultiplexing section 110 for each antenna into a time waveform and CPadding sections 112-1 and 112-2 add a CP to the time waveform to obtainOFDM signals.

Transmission RF sections 113-1 and 113-2 perform radio processing fortransmission (for example, up-conversion or digital-analog (D/A)conversion) on the OFDM signals inputted from CP adding sections 112-1and 112-2 and transmit the resultant received signals via antennas 114-1and 114-2.

On the other hand, RF receiving section 115 also performs radioprocessing for reception (for example, down-conversion or analog-digital(A/D) conversion) on radio signals received via antenna 114-1 at areceiving band and outputs the resultant received signals to CP removingsection 116.

CP removing section 116 removes the CP from the received signals andfast Fourier transform (FFT) section 117 converts the received signalsfrom which the CP is removed into frequency-domain signals.

Extracting section 118 extracts uplink data from the frequency-domainsignals received from FFT section 117 on the basis of uplink resourceallocation information received from control section 102 and InverseDiscrete Fourier transform (IDFT) section 119 converts the extractedsignals into time-domain signals and outputs the time-domain signals todata receiving section 120 and ACK/NACK receiving section 121.

Data receiving section 120 decodes the time-domain signals inputted fromIDFT section 119. Data receiving section 120 then outputs decoded uplinkdata as received data.

ACK/NACK receiving section 121 extracts, from the time-domain signalsreceived from IDFT section 119, ACK/NACK signals from each terminal forthe downlink data (PDSCH signals). Specifically, ACK/NACK receivingsection 121 extracts the ACK/NACK signals from an uplink control channel(e.g., a Physical Uplink Control Channel (PUCCH) on the basis of theinformation inputted from allocating section 106. The uplink controlchannel is associated with the CCEs used for the transmission of thedownlink allocation control information corresponding to the downlinkdata.

ACK/NACK receiving section 121 then determines the ACK or NACK of theextracted ACK/NACK signals.

One reason that the CCEs and the PUCCH are associated with each other isto obviate the need for signaling sent by the base station to notifyeach terminal of a PUCCH for use in transmitting ACK/NACK signals fromthe terminal, which thereby allows downlink communication resources tobe efficiently used. Consequently, in accordance with the associationbetween the CCEs and the PUCCH, each terminal determines a PUCCH for usein transmitting ACK/NACK signals on the basis of the CCEs to whichdownlink allocation control information (DCI) for the terminal ismapped.

[Configuration of Terminal 200]

FIG. 8 is a block diagram illustrating the configuration of terminal 200according to Embodiment 1 of the present invention. Terminal 200receives downlink data and transmits an ACK/NACK signal corresponding tothe downlink data to base station 100 using a PUCCH which is an uplinkcontrol channel.

In terminal 200 shown in FIG. 8, RF receiving section 202 configures areception band based on band information received from configurationinformation receiving section 206. RF reception section 202 performsradio processing for reception (e.g., down-conversion or analog-digital(A/D) conversion) on radio signals (i.e., OFDM signals in this case)received via antenna 201 at the reception band and outputs resultantreceived signals to CP removing section 203. The received signals mayinclude a PDSCH signal, DCI, and upper-layer control informationincluding configuration information and search space information. TheDCI (allocation control information) for terminal 200 is allocated to acommon search space (C-SS) configured for terminal 200 and otherterminals or to a specific search space (UE-SS) configured for terminal200.

CP removing section 203 removes a CP from the received signals and FFTsection 204 converts the received signals from which the CP is removedinto frequency-domain signals. The frequency-domain signals areoutputted to demultiplexing section 205.

Demultiplexing section 205 outputs a component of signals received fromFFT section 204 that may include DCI (i.e., signals extracted from aPDCCH region and an R-PDCCH region) to PDCCH receiving section 207.Demultiplexing section 205 also outputs upper-layer control signals(e.g., RRC signaling) including configuration information toconfiguration information receiving section 206 and data signals (i.e.,PDSCH signals) to PDSCH receiving section 208.

Configuration information receiving section 206 reads, from theupper-layer control signals inputted from demultiplexing section 205,band information configured for the terminal, information indicating aterminal ID configured for the terminal, search space informationconfigured for the terminal, information indicating a reference signalconfigured for the terminal and information indicating a transmissionmode configured for the terminal.

The band information configured for the terminal is outputted to PDCCHreceiving section 207, RF receiving section 202 and RF transmittingsection 215. The information indicating a terminal ID set for theterminal is outputted to PDCCH receiving section 207 as terminal IDinformation. The search space information configured for the terminal isoutputted to PDCCH receiving section 207 as search space regioninformation. The information indicating a reference signal set for theterminal is outputted to PDCCH receiving section 207 as reference signalinformation. The information indicating a transmission mode configuredfor the terminal is outputted to PDCCH receiving section 207 astransmission mode information.

PDCCH receiving section 207 blind-decodes (monitors) the DCI inputtedfrom demultiplexing section 205 to obtain DCI for the terminal. PDCCHreceiving section 207 performs blind-decoding for a DCI format for theallocation of data common to all the terminals (for example, DCI 0/1A),a DCI format dependent on the transmission mode configured for theterminal (for example, one of DCI 1, 1B, 1D, 2, 2A, 2B, 2C, 0A, 0B and4), and a DCI format for the allocation of channels common to all theterminals (for example, DCI 1C and 1A). This operation creates DCIcontaining allocation control information on the DCI formats.

To be more specific, PDCCH receiving section 207 blind-decodes a C-SSindicated by the search space region information inputted fromconfiguration information receiving section 206 in a DCI format for theallocation of a common channel (DCI 1C and 1A) and in a DCI format (DCI0/1A) for the allocation of data common to all the terminals. That is,for each region candidate targeted for blind decoding in a C-SS (i.e.,candidates of a CCE region allocated to terminal 200), PDCCH receivingsection 207 demodulates and decodes the size of the DCI format forcommon channel allocation and the size of the DCI format for theallocation of data common to all the terminals. For the decoded signals,PDCCH receiving section 207 demasks CRC bits with an ID common to aplurality of terminals. PDCCH receiving section 207 then determinessignals for which “CRC-=0K” is found (i.e. no error is found) as aresult of the demasking to be DCI containing allocation controlinformation for a common channel. For the decoded signals, PDCCHreceiving section 207 further demasks the CRC bits with the ID of theterminal indicated by the terminal ID information. PDCCH receivingsection 207 then determines signals for which “CRC=OK” is found (i.e. noerror is found) as a result of the demasking to be DCI containingallocation control information for the terminal. In other words, PDCCHreceiving section 207 determines, in a C-SS, whether allocation controlinformation on DCI 0/1A is for a common channel or for the allocation ofdata to the terminal with a terminal ID (i.e., an ID common to aplurality of terminals or the ID of terminal 200).

PDCCH receiving section 207 calculates a UE-SS for the terminal for eachCCE aggregation level with the terminal ID indicated by the terminal IDinformation inputted from configuration information receiving section206. For each blind decoding region candidate (CCE candidate of each CCEaggregation level) in the obtained UE-SS, PDCCH receiving section 207then demodulates and decodes the size of the DCI format corresponding tothe transmission mode configured for the terminal (the transmission modeindicated by the transmission mode information) and the size of the DCIformat common to all the terminals (DCI 0/1A). For the decoded signals,PDCCH receiving section 207 demasks CRC bits with the ID of theterminal. PDCCH receiving section 207 determines signals for which“CRC=OK” is found (i.e. no error is found) as a result of demasking tobe DCI for the terminal.

Even if the search space region indicated by the search space regioninformation inputted from configuration information receiving section206 includes an R-PDCCH region, PDCCH receiving section 207blind-decodes (monitors) search spaces configured for the PDCCH regionand R-PDCCH region and acquires DCI for the terminal transmitted usingthe PDCCH and R-PDCCH as in the case of the aforementioned PDCCH region.

If PDCCH receiving section 207 receives no search space regioninformation (i.e., if base station 100 transmits no search space regioninformation) from configuration information receiving section 206, PDCCHreceiving section 207 may perform blind decoding in a transmissionregion of a plurality of items of DCI which may be directed to terminal200 without considering the search spaces of terminal 200.

Upon reception of downlink allocation control information, PDCCHreceiving section 207 outputs downlink resource allocation informationcontained in the DCI for the terminal to PDSCH receiving section 208 andupon reception of uplink allocation control information, PDCCH receivingsection 207 outputs uplink resource allocation information to mappingsection 212. PDCCH receiving section 207 also outputs the CCE number forthe CCE used for the transmission of the DCI for the terminal (i.e., CCEused for the transmission of the signals for which “CRC=OK” is found) tomapping section 212 (CCE number for the leading CCE if the CCEaggregation level is plural). The details of blind decoding (monitoring)in the PDCCH receiving section 207 will be described later.

PDSCH receiving section 208 extracts received data (i.e., downlink data)from the PDSCH signals inputted from demultiplexing section 205 on thebasis of the downlink resource allocation information received fromPDCCH receiving section 207. That is, PDSCH receiving section 208receives downlink data (downlink data signal) based on downlink resourceallocation information (allocation control information) for terminal 200allocated to one of a plurality of DCI allocation region candidates(blind decoding region candidates). PDSCH receiving section 208 alsodetects any error in the extracted received data (i.e., downlink data).If an error is found in the received data as a result of the errordetection, PDSCH receiving section 208 generates NACK signals asACK/NACK signals. If no error is found in the received data, PDSCHreceiving section 208 generates ACK signals as ACK/NACK signals. TheACK/NACK signals are outputted to modulating section 209.

Modulating section 209 modulates the ACK/NACK signals inputted fromPDSCH receiving section 208 and outputs the modulated ACK/NACK signalsto mapping section 212.

Modulating section 210 modulates transmission data (i.e., uplink data)and outputs the modulated data signal to DFT section 211.

DFT section 211 converts the data signals received from modulatingsection 210 into the frequency-domain and outputs a plurality ofresultant frequency components to mapping section 212.

Mapping section 212 maps the frequency component corresponding to thedata signal among a plurality of frequency components inputted from DFTsection 211 to a PUSCH in accordance with the uplink resource allocationinformation inputted from PDCCH receiving section 207. Mapping section212 also identifies a PUCCH in accordance with the CCE number inputtedfrom PDCCH receiving section 207. Mapping section 212 then maps theACK/NACK signals inputted from modulating section 209 to the identifiedPUCCH.

IFFT section 213 converts the plurality of frequency components mappedto the PUSCH into a time-domain waveform. CP adding section 214 adds aCP to the time-domain waveform.

RF transmitting section 215 can vary the range for transmission. RFtransmitting section 215 determines a specific transmission range on thebasis of the band information received from configuration informationreceiving section 206. RF transmitting section 215 then performstransmission radio processing (for example, up-conversion ordigital-analog (D/A) conversion) on the CP-added signals and transmitsthe resultant signals via antenna 201

[Operations of Base Station 100 and Terminal 200]

Operations of base station 100 and terminal 200 having theabove-described configurations will be described with reference to FIG.9A and FIG. 9B.

In the following description, 6 DCI allocation region candidates (thatis, blind decoding region candidates) in total are configured as C-SSs;4 candidates (16 CCEs (=4 CCEs×4 candidates)) for CCE aggregation level4 and 2 candidates (16 CCEs (=8 CCEs×2 candidates)) for CCE aggregationlevel 8. Furthermore, 16 DCI allocation region candidates (that is,blind decoding region candidates) in total are configured as UE-SSs; 6candidates (6 CCEs (=1 CCE×6 candidates)), 6 candidates (12 CCEs (=2CCEs×6 candidates)), 2 candidates (8 CCEs (4 CCEs×candidates)) and 2candidates (16 CCEs (=8 CCEs×2 candidates)) for CCE aggregation levels1, 2, 4 and 8 respectively. That is, search spaces (C-SS and UE-SS)composed of 22 DCI allocation region candidates (that is, blind decodingregion candidates) in total are configured for each terminal.

Furthermore, a case will be described here where configuration section101 of base station 100 configures both the PDCCH region and the R-PDCCHregion for terminal 200 as PDCCH transmission regions. In this case, theabove 22 DCI allocation region candidates (that is, blind decodingregion candidates) are each configured in one of the PDCCH region and anR-PDCCH region. In other words, the sum of the number of DCI allocationregion candidates configured in a PDCCH region and the number of DCIallocation region candidates configured in an R-PDCCH region amounts to22.

Furthermore, configuration section 101 configures a CRS as a referencesignal for use in a PDCCH region and configures one of a CRS and a DM-RSas a reference signal for use in an R-PDCCH region.

Search space configuration section 103 configures a common search space(C-SS) and a specific search space (UE-SS) based on a PDCCH transmissionregion indicated by the configuration information inputted fromconfiguration section 101 and reference signals used.

To be more specific, when using both a PDCCH region and an R-PDCCHregion as a region for transmitting a PDCCH to terminal 200, searchspace configuration section 103 configures a search space in the PDCCHregion (C-SS or UE-SS) and a search space (C-SS or UE-SS) in the R-PDCCHregion respectively. In this case, search space configuration section103 configures search spaces for each terminal so that the ratio of DCIallocation region candidates in the R-PDCCH region to the DCI allocationregion candidates in the PDCCH region is greater in a terminal for whicha DM-RS is indicated as a reference signal for demodulation in theR-PDCCH region (terminal using DM-RSs in the R-PDCCH region) than in aterminal for which a CRS is indicated as a reference signal fordemodulation in the R-PDCCH region (terminal using CRSs in the R-PDCCHregion).

When using only an R-PDCCH region as a region for transmitting a PDCCHto terminal 200, search space configuration section 103 configuressearch spaces (C-SS and UE-SS) of terminal 200 in the PDCCH region.

For example, as shown in FIG. 9A, for a certain terminal using CRSs inthe R-PDCCH region, search space configuration section 103 configuresUE-SSs of 12 candidates in total in the PDCCH region; 6 candidates ofCCE aggregation level 2 and 6 candidates of CCE aggregation level 1.Furthermore, as shown in FIG. 9B, for a certain terminal using CRSs inthe R-PDCCH region, search space configuration section 103 configuresC-SSs of 6 candidates in total in the R-PDCCH region; 2 candidates ofCCE aggregation level 8 and 4 candidates of CCE aggregation level 4, andUE-SSs of 4 candidates in total; 2 candidates of CCE aggregation level 8and 2 candidates of CCE aggregation level 4. Here, as shown in FIG. 10,if the number of DCI allocation region candidates in the PDCCH regionfor a terminal using CRSs in the R-PDCCH region is assumed to be N_(A)and the number of DCI allocation region candidates in the R-PDCCH regionis assumed to be N_(B), N_(A)=12 and N_(B)=10 in FIG. 9A and FIG. 9Brespectively.

On the other hand, as shown in FIG. 9A, for a certain terminal usingDM-RSs in an R-PDCCH region, search space configuration section 103configures C-SSs of 6 candidates in total in a PDCCH region; 2candidates of CCE aggregation level 8 and 4 candidates of CCEaggregation level 4, and configures UE-SSs of 4 candidates in total; 2candidates of CCE aggregation level 8 and 2 candidates of CCEaggregation level 4. Furthermore, as shown in FIG. 9B, for a certainterminal using DM-RSs in an R-PDCCH region, search space configurationsection 103 configures UE-SSs of 12 candidates in total in an R-PDCCHregion; 6 candidates of CCE aggregation level 2 and 6 candidates of CCEaggregation level 1. Here, as shown in FIG. 10, if the number of DCIallocation region candidates in a PDCCH region for a terminal usingDM-RSs in the R-PDCCH region is assumed to be N_(C) and the number ofDCI allocation region candidates in the R-PDCCH region is assumed to beN_(D), then N_(C)=10 and N_(D)=12 in FIG. 9A and FIG. 9B respectively.

That is, search space configuration section 103 configures search spaces(C-SS and UE-SS) so that a ratio of the number of DCI allocation regioncandidates N_(D) in the R-PDCCH region to the number of DCI allocationregion candidates N_(C) in the PDCCH region (N_(D)/N_(C)) in terminal200 using DM-RSs in the R-PDCCH region is equal to or above a ratio ofthe number of DCI allocation region candidates N_(B) in the R-PDCCHregion to the number of DCI allocation region candidates N_(A) in thePDCCH region (N_(B)/N_(A)) in terminal 200 using CRSs in the R-PDCCHregion. That is, search space configuration section 103 configures theallocation of search spaces between the PDCCH region and R-PDCCH regionso as to satisfy (N_(B)/N_(A)≦N_(D)/N_(C)).

In other words, the ratio of the number of blind decoding regioncandidates N_(D) in the R-PDCCH region to the number of blind decodingregion candidates N_(C) in the PDCCH region (N_(D)/N_(C)) in terminal200 using DM-RSs in the R-PDCCH region is equal to or above the ratio ofthe number of blind decoding region candidates N_(B) in the R-PDCCHregion to the number of blind decoding region candidates N_(A) in thePDCCH region (N_(B)/N_(A)) in terminal 200 using CRSs in the R-PDCCHregion.

Allocating section 106 then allocates DCI containing allocation controlinformation to CCEs in C-SSs or UE-SSs shown in FIG. 9A and FIG. 9B.

To be more specific, allocating section 106 allocates DCI containingallocation control information for common channels (e.g., DCI 1C and 1A)to CCEs in C-SSs shown in FIG. 9A and FIG. 9B. Furthermore, allocatingsection 106 allocates DCI containing allocation control information forthe allocation of data common to all the terminals (e.g., DCI 0/1A) toCCEs in CSSs or CCEs in UE-SSs shown in FIG. 9A and FIG. 9B.Furthermore, allocating section 106 allocates DCI containing allocationcontrol information dependent on the transmission mode configured in theterminal (e.g., uplink (DCI 0A, 0B and 4) and downlink (DCI 1, 1B, 1D,2, 2A, 2B and 2C)) to CCEs in UE-SSs shown in FIG. 9A and FIG. 9B.

In contrast, in terminal 200, PDCCH receiving section 207 blind-decodesDCI containing allocation control information for common channels (e.g.,DCI 1C and 1A) and DCI containing allocation control information for theallocation of data common to all the terminals (e.g., DCI 0/1A) forC-SSs shown in FIG. 9A and FIG. 9B. Furthermore, for UE-SSs shown inFIG. 9A and FIG. 9B, PDCCH receiving section 207 blind-decodes DCIcontaining allocation control information for the allocation of datacommon to all the terminals (e.g., DCI 0/1A) and DCI containingallocation control information dependent on the transmission mode (e.g.,uplink (DCI 0A, 0B and 4), downlink (DCI 1, 1B, 1D, 2, 2A, 2B and 2C))configured for terminal 200. That is, terminal 200 performs blinddecoding on two DCI formats (DCI 1C, 1A and DCI 0/1A) for C-SSs. On theother hand, terminal 200 performs blind decoding on three DCI formats(DCI dependent on the uplink transmission mode (DCI 0A, 0B and 4), DCIdependent on the downlink transmission mode (DCI 1, 1B, 1D, 2, 2A, 2Band 2C) and DCI 0/1A) for UE-SSs.

Blind decoding in terminal 200 described above is summarized as follows:36 (=12 candidates×3 DCI formats) blind decoding operations in total areperformed for UE-SSs of 12 candidates in a PDCCH region (FIG. 9A) for aterminal using CRSs in the R-PDCCH region. Furthermore, 12 (=6candidates×2 DCI formats) blind decoding operations in total areperformed for C-SSs of 6 candidates and 12 (=4 candidates×3 DCI formats)blind decoding operations in total are performed for UE-SSs of 4candidates in the R-PDCCH region (FIG. 9B).

Furthermore, a terminal using DM-RSs in the R-PDCCH region performs 12(=6 candidates×2 DCI formats) blind decoding operations in total forC-SSs of 6 candidates in the PDCCH region (FIG. 9A) and 12 (=4candidates×3 DCI formats) blind decoding operations in total for UE-SSsof 4 candidates. Furthermore, the terminal performs 36 (=12 candidates×3DCI formats) blind decoding operations for UE-SSs of 12 candidates inthe R-PDCCH region (FIG. 9B).

That is, the ratio of the number of blind decoding operations in theR-PDCCH region to the number of blind decoding operations in the PDCCHregion of the terminal using DM-RSs in the R-PDCCH region is greaterthan that of the terminal using CRSs in the R-PDCCH region. To be morespecific, in terminal 200, the ratio (36/24) of the number of blinddecoding operations in the R-PDCCH region (36 in FIG. 9B) to the numberof blind decoding operations in the PDCCH region (24 in FIG. 9A) interminal 200 using DM-RSs in the R-PDCCH region is equal to or greaterthan the ratio (24/36) of the number of blind decoding operations in theR-PDCCH region (24 in FIG. 9B) to the number of blind decodingoperations in the PDCCH region (36 in FIG. 9A) in terminal 200 usingCRSs in the R-PDCCH region.

Thus, more DCI allocation region candidates (blind decoding regioncandidates) are more likely to be configured in an R-PDCCH region for aterminal using DM-RSs in the R-PDCCH region than a terminal using CRSsin the R-PDCCH region. That is, with the terminal using DM-RSs in theR-PDCCH region, DCI is more likely to be allocated to the R-PDCCHregion.

Here, as shown in FIG. 3, use of DM-RSs is effective for a terminallocated at a cell edge or a terminal whose received signal level is low.That is, base station 100 is more likely to instruct terminal 200located at a cell edge and receiving large interference from other cells(other cell interference) (or terminal 200 whose received signal levelis low) to use DM-RSs in the R-PDCCH region.

Furthermore, an R-PDCCH region is more likely to be able to reduce othercell interference than a PDCCH region through, for example, interferencecontrol.

Thus, base station 100 configures search spaces so that DCI is morelikely to be allocated to a terminal using DM-RSs (e.g., terminal 200located near a cell edge) in an R-PDCCH region. In this way, theterminal using DM-RSs achieves an effect of improving received signalpower of a PDCCH using DM-RSs (see FIG. 3) while reducing influences ofother cell interference, and can thereby secure desired receivingquality.

Here, for example, base station 100 allocates data for a terminal usingCRSs in an R-PDCCH region within a search space using relatively moreCCEs in a PDCCH region (that satisfies, for example, N_(B)≧N_(A)) so asto satisfy a relationship of (N_(B)/N_(A)≦N_(D)/N_(C)). On the otherhand, base station 100 allocates data for a terminal using DM-RSs in anR-PDCCH region within a search space using relatively more CCEs in anR-PDCCH region (that satisfies, for example, N_(C)≦N_(D)).

This makes it possible to achieve a load balance of PDCCHs to whichcontrol information for each terminal is allocated, that is, touniformly map PDCCHs onto a PDCCH region and an R-PDCCH region. That is,it is possible to avoid cases where PDCCHs cannot be transmitted withina limited region because PDCCHs are biased to a specific region. Thus,it is possible to prevent one of the PDCCH region and R-PDCCH regionfrom becoming tight as the number of communicating terminals increases.Furthermore, preventing the R-PDCCH region from becoming tight makes itpossible to avoid an adverse influence of data allocation on a relaystation (a terminal under the control of a relay station) and preventthe PDSCH region from becoming tight. This prevents system throughputfrom deteriorating.

Furthermore, each terminal moves in a cell covered by base station 100.For example, in a cell covered by base station 100, there are terminalsin a variety of situations such as a terminal moving fast, terminalmoving slowly, terminal located near a cell edge or terminal locatednear the center of the cell. For such terminals, base station 100 mayuse a PDCCH region or R-PDCCH region using CRSs (reference signals withwhich a frequency diversity effect can be achieved) for DCI for theterminal moving fast (whose received signal level fluctuates violently).Furthermore, base station 100 may use an R-PDCCH region using DM-RSs(reference signals with which received signal power can be improved) forDCI for the terminal located near a cell edge (whose received signallevel is low). That is, using a PDCCH region and an R-PDCCH region asthe PDCCH transmission region, base station 100 can configure searchspaces capable of securing desired receiving quality regardless of themoving speed or location area or the like of a terminal.

Thus, base station 100 determines which region (PDCCH region or R-PDCCHregion) is used to transmit DCI in consideration of the situation ofeach terminal (e.g., position of the terminal, scale of other cellinterference, traffic situation (e.g., “the PDCCH region becomes tightas the number of communicating terminals increases”) or the like).Furthermore, when allocating DCI for a terminal to an R-PDCCH region,base station 100 configures one of CRS and DM-RS as a reference signalfor demodulation in the R-PDCCH region. In this case, base station 100configures search spaces in the PDCCH region and R-PDCCH region so as tosatisfy the above-described relationship of (N_(B)/N_(A)≦N_(D)/N_(C)).

Thus, it is possible to prevent throughput deterioration in the entiresystem caused by the PDCCH region or R-PDCCH region becoming tight.Furthermore, each terminal can receive DCI using a resource(transmission region of a PDCCH) suited to the situation of eachterminal and secure desired receiving quality in the terminal.

Therefore, according to the present embodiment, even when DCI for aterminal connected to a base station is allocated to a PDCCH region andan R-PDCCH region, it is possible to secure desired receiving quality inthe terminal without causing system throughput to deteriorate.

In the present embodiment, when a terminal using CRSs in an R-PDCCHregion and a terminal using DM-RSs in an R-PDCCH region coexist, basestation 100 configures search spaces in the PDCCH region and R-PDCCHregion so as to satisfy the aforementioned relationship of(N_(B)/N_(A)≦N_(D)/N_(C)). However, when a terminal using CRSs in anR-PDCCH region and a terminal using DM-RSs in an R-PDCCH region coexist,base station 100 may also cause the number of DCI allocation regioncandidates (number of blind decoding region candidates) N_(B) in theR-PDCCH region for a terminal using CRSs in the R-PDCCH region to beequal to or below the number of DCI allocation region candidates (numberof blind decoding region candidates) N_(D) (N_(B)≦N_(D)) in the R-PDCCHregion for a terminal using DM-RSs in the R-PDCCH region. Here, if thenumber of DCI allocation region candidates contained in a search spaceconfigured for each terminal is assumed to be constant (e.g., 22candidates in FIG. 9A and FIG. 9B), satisfying the relationship of(N_(B)≦N_(D)) is equivalent to satisfying the relationship of(N_(B)/N_(A)≦N_(D)/N_(C)) as in the case of the present embodiment.

Similarly, base station 100 may also cause the number of DCI allocationregion candidates (number of blind decoding region candidates) N_(A) inthe PDCCH region for a terminal using CRSs in the R-PDCCH region to beequal to or above the number of DCI allocation region candidates (numberof blind decoding region candidates) N_(C) (N_(A)≧N_(C)) in the PDCCHregion for a terminal using DM-RSs in the R-PDCCH region. In this case,if the number of DCI allocation region candidates contained in a searchspace configured for each terminal is also assumed to be constant (e.g.,22 candidates in FIG. 9A and FIG. 9B), satisfying the relationship of(N_(A)≧N_(C)) is equivalent to satisfying the relationship of(N_(B)/N_(A)≦N_(D)/N_(C)) as in the case of the present embodiment.

Alternatively, in the present embodiment, base station 100 may alsoconfigure search spaces of each terminal so as to satisfy therelationship of (N_(A)≧N_(C)) and satisfy the relationship of(N_(B)≦N_(D)) at the same time. At this time, the ratio between N_(A)and N_(B), and the ratio between N_(C) and N_(D) can take any value. Forexample, the ratio between N_(C) and N_(D) (N_(C):N_(D)) may beN_(C):N_(D)=12:10 or N_(C):N_(D)=10:12.

Alternatively, in the present embodiment, base station 100 may alsodispose C-SSs in a PDCCH region and dispose UE-SSs in an R-PDCCH region,for example, so as to satisfy the relationship of (N_(A)≧N_(C)) and therelationship of (N_(B)≦N_(D)) at the same time.

For example, base station 100 configures search spaces as shown in FIG.9A and FIG. 9B, resulting in N_(A)=12, N_(B)=10, N_(C)=10 and N_(D)=12,which satisfies (N_(A)≧N_(C)) and (N_(B)≦N_(D)). That is, base station100 configures search spaces so that in the R-PDCCH region, the numberof DCI allocation region candidates (number of blind decoding regioncandidates) N_(D) for a terminal using DM-RSs is greater than the numberof DCI allocation region candidates (number of blind decoding regioncandidates) N_(B) for a terminal using CRSs. As in the case of thepresent embodiment, it is possible for a terminal using DM-RSs to obtainan effect of improving received signal power of a PDCCH using DM-RSs(see FIG. 3) while suppressing influences of other cell interference.

Furthermore, when (N_(A)≧N_(C)) and (N_(B)≦N_(D)) are satisfied, thenumber of DCI allocation region candidates (number of blind decodingregion candidates) N_(B) for a terminal using CRSs becomes smaller thanthe number of DCI allocation region candidates (number of blind decodingregion candidates) N_(D) for a terminal using DM-RSs in the R-PDCCHregion, whereas in the PDCCH region, the number of DCI allocation regioncandidates (number of blind decoding region candidates) N_(A) for aterminal using CRSs becomes greater than the number of DCI allocationregion candidates (number of blind decoding region candidates) N_(C) fora terminal using DM-RSs. This makes it possible to maintain a loadbalance of PDCCHs in the PDCCH region and R-PDCCH region as a whole.

Embodiment 2

A case has been described in Embodiment 1 where a terminal using CRSsand a terminal using DM-RSs coexist in an R-PDCCH region. In contrast, acase will be described in the present embodiment where reference signalsused for each terminal are fixedly configured in an R-PDCCH region.

Hereinafter, search space configuration methods 1 to 3 according to thepresent embodiment will be described more specifically.

Since the basic configurations of a base station and a terminalaccording to the present embodiment are common to those in Embodiment 1,these configurations will be described using FIG. 6 and FIG. 8.

In the following description, as in the case of Embodiment 1, 6 DCIallocation region candidates (that is, blind decoding region candidates)in total are configured as C-SSs; 4 candidate (16 CCEs) for CCEaggregation level 4 and 2 candidates (16 CCEs) for CCE aggregation level8. Furthermore, 16 DCI allocation region candidates (that is, blinddecoding region candidates) in total are configured as UE-SSs; 6candidates (6 CCEs), 6 candidates (12 CCEs), 2 candidates (8 CCEs), 2candidates (16 CCEs) corresponding to CCE aggregation levels 1, 2, 4 and8 respectively. That is, search spaces (C-SS and UE-SS) composed of 22DCI allocation region candidates in total (that is, blind decodingregion candidates) are configured for each terminal.

Furthermore, as in the case of Embodiment 1, for C-SSs, terminal 200(PDCCH receiving section 207) performs blind decoding for two DCIformats; DCI containing allocation control information for commonchannels (e.g., DCI 1C and 1A) and DCI containing allocation controlinformation for the allocation of data common to all the terminals(e.g., DCI 0/1A). For UE-SSs, terminal 200 (PDCCH receiving section 207)performs blind decoding for three DCI formats; DCI containing allocationcontrol information for the allocation of data common to all theterminals (for example, DCI 0/1A), DCI containing allocation controlinformation dependent on the transmission mode configured for terminal200 (for example, uplink (DCI 0A, 0B and 4), and downlink (DCI 1, 1B,1D, 2, 2A, 2B and 2C)). Furthermore, as in the case of Embodiment 1, asshown in FIG. 10, the number of DCI allocation region candidates (numberof blind decoding region candidates) in a PDCCH region for a terminalusing CRSs in an R-PD CCH region is assumed to be N_(A) and the numberof DCI allocation region candidates (number of blind decoding regioncandidates) in an R-PDCCH region is assumed to be N_(B). Furthermore, asshown in FIG. 10, the number of DCI allocation region candidates (numberof blind decoding region candidates) in a PDCCH region for a terminalusing DM-RSs in an R-PDCCH region is assumed to be N_(C) and the numberof DCI allocation region candidates (number of blind decoding regioncandidates) in an R-PDCCH region is assumed to be N_(D).

<Configuration Method 1 (when Configuring Only CRSs in R-PDCCH Region)>

According to configuration method 1, search space configuration section103 of base station 100 configures search spaces so that the number ofDCI allocation region candidates (number of blind decoding regioncandidates) N_(B) in R-PDCCH region becomes equal to or above the numberof DCI allocation region candidates (number of blind decoding regioncandidates) N_(A) in a PDCCH region (N_(A)≦N_(B)).

For example, as shown in FIG. 11, for a certain terminal, search spaceconfiguration section 103 configures C-SSs of 6 candidates in total in aPDCCH region; 2 candidates of CCE aggregation level 8 and 4 candidatesof CCE aggregation level 4 and configures UE-SSs of 4 candidates intotal; 2 candidates of CCE aggregation level 8 and 2 candidates of CCEaggregation level 4. Furthermore, as shown in FIG. 11, for a certainterminal, search space configuration section 103 configures UE-SSs of 12candidates in total in an R-PDCCH region; 6 candidates of CCEaggregation level 2 and 6 candidate of CCE aggregation level 1. Thus,the number of DCI allocation region candidates (number of blind decodingregion candidates) N_(A) in the PDCCH region shown in FIG. 11 is 10 andthe number of DCI allocation region candidates (number of blind decodingregion candidates) N_(B) in the R-PDCCH region is 12. That is, in FIG.11, the relationship of (N_(A)≦N_(B)) is satisfied.

In contrast, PDCCH receiving section 207 of terminal 200 blind-decodesDCI for the C-SSs and UE-SSs shown in FIG. 11 according to search spaceregion information inputted from configuration information receivingsection 206. In FIG. 11, PDCCH receiving section 207 blind decodes foronly C-SSs of CCE aggregation levels 4 and 8 and for only UE-SSs of CCEaggregation levels 4 and 8 in the PDCCH region, and blind decodes foronly UE-SSs of CCE aggregation levels 1 and 2 in the R-PDCCH region.

That is, PDCCH receiving section 207 performs 12 (=6 candidates×2 types)blind decoding operations for C-SSs configured for the PDCCH regionshown in FIG. 11 and performs (=4 candidates×3 types) blind decodingoperations for UE-SSs configured for the PDCCH region shown in FIG. 11.On the other hand, PDCCH receiving section 207 performs 36 (=12candidates×3 types) blind decoding operations for UE-SSs configured forthe R-PDCCH region shown in FIG. 11.

That is, in FIG. 11, the number of blind decoding operations (36operations) in the R-PDCCH region is greater than the number of blinddecoding operations (24 operations) in the PDCCH region.

Here, according to configuration method 1, the same reference signal(CRS) is used for terminal 200 in both the PDCCH region and R-PDCCHregion. That is, there is no difference in reference signals used in thePDCCH region and R-PDCCH region for terminal 200 for which the searchspaces shown in FIG. 11 are configured. On the other hand, as describedabove, other cell interference is more likely to be reduced in theR-PDCCH region than in the PDCCH region.

Therefore, as shown in FIG. 11, base station 100 configures searchspaces so that the number of DCI allocation region candidates (number ofblind decoding region candidates) N_(B) in the R-PDCCH region is greaterthan the number of DCI allocation region candidates (number of blinddecoding region candidates) N_(A) in the PDCCH region (N_(A)≦N_(B)), andterminal 200 is thereby more likely to be able to receive controlinformation allocated to terminal 200 in the R-PDCCH region where othercell interference is more likely to be reduced.

Furthermore, as shown in FIG. 11, by using the PDCCH region and R-PDCCHregion as PDCCH transmission regions for terminal 200, base station 100can select a transmission region to transmit DCI depending on the movingspeed (high-speed or low-speed) of terminal 200 or the location area(center of the cell or cell edge) of terminal 200. That is, base station100 can configure search spaces capable of securing desired receivingquality regardless of the moving speed or location area or the like ofthe terminal.

Thus, according to configuration method 1, when DCI for a terminalconnected to a base station is allocated to a PDCCH region and anR-PDCCH region, it is possible to secure desired receiving quality inthe terminal by further reducing the possibility of receiving other cellinterference. Furthermore, robust PDCCH reception performance can besecured depending on the situation of the terminal.

Configuration method 1 may satisfy not only the configuration of searchspaces (allocation of search spaces) shown in FIG. 11 but also therelationship of (N_(A)≦N_(B)). For example, UE-SSs of 10 candidates(e.g., 10 candidates in total of CCE aggregation levels 2, 4 and 8 among16 candidates) may be configured in the PDCCH region (N_(A)=10) andC-SSs of 6 candidates and UE-SSs of 6 candidates (6 candidates of CCEaggregation level 1); 12 candidates in total, may be configured in theR-PDCCH region (N_(B)=12). Alternatively, C-SSs of 6 candidates may beconfigured in the PDCCH region (N_(A)=6) and UE-SSs of 16 candidates (16candidates in total of CCE aggregation levels 1, 2, 4 and 8) may beconfigured in the R-PDCCH region (N_(B)=16).

<Configuration Method 2 (when Configuring Only DM-RSs in R-PDCCHRegion)>

According to configuration method 2, search space configuration section103 of base station 100 configures search spaces so that the number ofDCI allocation region candidates (number of blind decoding regioncandidates) N_(D) in an R-PDCCH region is equal to or above the numberof DCI allocation region candidates (number of blind decoding regioncandidates) N_(C) in a PDCCH region (N_(C)≦N_(D)).

For example, as in the case of configuration method 1, as shown in FIG.11, for a certain terminal, search space configuration section 103configures C-SSs of 6 candidates in total in the PDCCH region; 2candidates of CCE aggregation level 8 and 4 candidates of CCEaggregation level 4, and configures UE-SSs of 4 candidates in total; 2candidates of CCE aggregation level 8 and 2 candidates of CCEaggregation level 4. Furthermore, as shown in FIG. 11, for a certainterminal, search space configuration section 103 configures UE-SSs of 12candidates in total in the R-PDCCH region; 6 candidates of CCEaggregation level 2 and 6 candidates of CCE aggregation level 1. Thus,the number of DCI allocation region candidates (number of blind decodingregion candidates) N_(C) in the PDCCH region shown in FIG. 11 is 10, andthe number of DCI allocation region candidates (number of blind decodingregion candidates) N_(D) in the R-PDCCH region is 12. That is, in FIG.11, the relationship of (N_(C)≦N_(D)) is satisfied.

In contrast, PDCCH receiving section 207 of terminal 200 blind-decodesDCI for the C-SSs and UE-SSs shown in FIG. 11 according to search spaceregion information inputted from configuration information receivingsection 206. In FIG. 11, PDCCH receiving section 207 performs blinddecoding only for C-SSs of CCE aggregation levels 4 and 8 and for UE-SSsof CCE aggregation levels 4 and 8 in the PDCCH region, and performsblind decoding only for UE-SSs of CCE aggregation levels 1 and 2 in theR-PDCCH region.

That is, as in the case of configuration method 1, PDCCH receivingsection 207 performs 12 (=6 candidates×2 types) blind decodingoperations for C-SSs configured in the PDCCH region shown in FIG. 11 andperforms 12 (=4 candidates×3 types) blind decoding operations forUE-SSs. Furthermore, as in the case of configuration method 1, PDCCHreceiving section 207 performs 36 (=12 candidates×3 types) blinddecoding operations for UE-SSs configured in the R-PDCCH region shown inFIG. 11.

That is, in FIG. 11, the number of blind decoding operations (36operations) in the R-PDCCH region is greater than the number of blinddecoding operations (24 operations) in the PDCCH region.

Here, according to configuration method 2, CRSs are used in the PDCCHregion and DM-RSs are used in the R-PDCCH region for terminal 200.Furthermore, as described above, use of DM-RSs is effective for aterminal located at a cell edge or the like or a terminal whose receivedsignal level is low. Furthermore, the R-PDCCH region is more likely tobe able to reduce other cell interference through interference controlor the like than the PDCCH region.

Thus, by making the number of blind decoding region candidates N_(D) inthe R-PDCCH region greater than the number of blind decoding regioncandidates N_(C) in the PDCCH region (N_(C)≦N_(D)), terminal 200 usingDM-RSs can obtain an effect of improving received signal power of aPDCCH using DM-RSs (see FIG. 3) while suppressing influences of othercell interference in the R-PDCCH region. Thus, desired receiving qualityin terminal 200 can be secured.

Furthermore, as shown in FIG. 11, by using the PDCCH region and R-PDCCHregion as PDCCH transmission regions for terminal 200, base station 100can select a transmission region using an appropriate reference signal(CRS or DM-RS) to transmit DCI depending on the moving speed (high-speedor low-speed) of terminal 200 or the location area (center of the cellor cell edge) of terminal 200. That is, base station 100 can configuresearch spaces capable of securing desired receiving quality regardlessof the moving speed or location area or the like of the terminal.

Thus, according to configuration method 2, when DCI for a terminalconnected to a base station is allocated to a PDCCH region and anR-PDCCH region, it is also possible to secure desired receiving qualityin the terminal by further reducing the possibility of receiving othercell interference. Furthermore, robust PDCCH reception performance canbe secured depending on the situation of the terminal.

A case has been described in configuration method 2 where only DM-RSsare configured in the R-PDCCH region. However, configuration method 2 islikewise applicable to a case where DM-RSs are indicated in a terminalin which CRSs or DM-RSs may be indicated in the R-PDCCH region.

At this time, in the case where CRSs are indicated in a terminal inwhich CRSs or DM-RSs may be indicated in the R-PDCCH region, searchspace configuration section 103 of base station 100 may configure searchspaces so that the number of DCI allocation region candidates (number ofblind decoding region candidates) N_(A) in the PDCCH region becomesequal to or above the number of DCI allocation region candidates (numberof blind decoding region candidates) N_(B) in the R-PDCCH region(N_(A)≧N_(B)). For example, for a certain terminal using CRSs in theR-PDCCH region, search space configuration section 103 configures UE-SSsof 12 candidates in total in the PDCCH region; 6 candidates of CCEaggregation level 2 and 6 candidates of CCE aggregation level 1(N_(A)=12). Furthermore, in the R-PDCCH region, search spaceconfiguration section 103 configures C-SSs of 6 candidates in total; 2candidates of CCE aggregation level 8 and 4 candidates of CCEaggregation level 4, and UE-SSs of 4 candidates in total; 2 candidatesof CCE aggregation level 8 and 2 candidates of CCE aggregation level 4(N_(B)=10).

In contrast, PDCCH receiving section 207 of terminal 200 performs 36(=12 candidates×3 types) blind decoding operations for UE-SSs in thePDCCH region. Furthermore, PDCCH receiving section 207 performs 12 (=6candidate×2 types) blind decoding operations for C-SSs in the R-PDCCHregion and performs 12 (−4 candidates×3 types) blind decoding operationsfor UE-SSs. Thus, the number of blind decoding operations (36operations) in the PDCCH region is greater than the number of blinddecoding operations (24 operations) in the R-PDCCH region.

Thus, as described above, it is possible to secure desired receivingquality for a terminal using DM-RSs and maintain a load balance ofPDCCHs in the PDCCH region and the R-PDCCH region as a whole.

<Configuration Method 3 (when Configuring Only DM-RSs in R-PDCCHRegion)>

According to configuration method 3, search space configuration section103 of base station 100 configures C-SSs only in a PDCCH region.

For example, for a certain terminal, search space configuration section103 configures C-SSs of 6 candidates in total in the PDCCH region; 2candidates of CCE aggregation level 8 and 4 candidates of CCEaggregation level 4. On the other hand, for a certain terminal, searchspace configuration section 103 arbitrarily configures 6 candidates, 6candidates, 2 candidates and 2 candidates of CCE aggregation levels 1,2, 4 and 8 respectively which are UE-SSs in the PDCCH region and R-PDCCHregion.

That is, search space configuration section 103 configures C-SSs of 6candidates in the PDCCH region and UE-SSs of (N_(C)−6) candidates andconfigures only UE-SSs of N_(D) candidates in the R-PDCCH region. Inthis case, the relationship of (N_(C)≦N_(D)) may be satisfied as in thecase of configuration method 2, for example.

Thus, PDCCH receiving section 207 of terminal 200 performs 12 (=6candidates×2 types) blind decoding operations for C-SSs in the PDCCHregion and ((N_(C)−12) candidates×3 types) blind decoding operations forUE-SSs. On the other hand, PDCCH receiving section 207 performs 0 blinddecoding operations (no blind decoding operations) for C-SSs in theR-PDCCH region and performs (N_(D) candidate×3 types) blind decodingoperations for UE-SSs.

Here, as shown in FIG. 3, precoding is not applied to reference signalsand data signals in the transmission region using CRSs, whereaspreceding is applied to reference signals and data signals in thetransmission region using DM-RSs (here, R-PDCCH region). Therefore, forexample, when C-SSs are configured in the R-PDCCH region using DM-RSs,precoding is also applied to the C-SSs with a transmission weightoptimized for a certain terminal. For this reason, there may be caseswhere other terminals cannot receive DCI allocated to CCEs in a C-SS.

In contrast, according to configuration method 3, search spaceconfiguration section 103 configures C-SSs only in a PDCCH region whereonly CRSs (reference signals without precoding) are used, and all theterminals can receive DCI allocated to CCEs in the C-SS.

Thus, according to configuration method 3, PDCCH reception performancein C-SSs for all the terminals can be secured.

A case has been described in configuration method 3 where only DM-RSsare configured in the R-PDCCH region. However, configuration method 3 islikewise applicable to a case where DM-RSs are indicated in a terminalin which CRSs or DM-RSs may be indicated in the R-PDCCH region. In thiscase, the relationship of (N_(B)/N_(A)≦N_(D)/N_(C)) may be satisfied asin the case of Embodiment 1.

Search space configuration methods 1 to 3 according to the presentembodiment have been described so far.

Thus, according to the present embodiment, when DCI for a terminalconnected to a base station is mapped onto a PDCCH region and an R-PDCCHregion, even when reference signals used for each terminal are fixedlyconfigured in the R-PDCCH region, desired receiving quality for theterminal can be secured in the PDCCH region or R-PDCCH region.

Embodiment 3

The present embodiment will describe a case where search spaces areconfigured for each DCI format.

Since basic configurations of a base station and a terminal according tothe present embodiment are common to those in Embodiment 1, theseconfigurations will be described using FIG. 6 and FIG. 8.

Furthermore, in the following description as in the case of Embodiment1, 6 DCI allocation region candidates (that is, blind decoding regioncandidates) in total are configured as C-SSs; 4 candidates (16 CCEs) forCCE aggregation level 4 and 2 candidates (16 CCEs) for CCE aggregationlevel 8. Furthermore, 16 DCI allocation region candidates (that is,blind decoding region candidates) in total are configured as UE-SSs; 6candidates (6 CCEs), 6 candidates (12 CCEs), 2 candidates (8 CCEs) and 2candidates (16 CCEs) for CCE aggregation levels 1, 2, 4 and 8respectively. That is, search spaces (C-SS and UE-SS) composed of 22 DCIallocation region candidates in total (that is, blind decoding regioncandidates) are configured for each terminal.

Furthermore, as in the case of Embodiment 1, terminal 200 (PDCCHreceiving section 207) performs blind decoding for C-SSs in two DCIformats; DCI containing allocation control information for commonchannels (e.g., DCI 1C and 1A) and DCI containing allocation controlinformation for the allocation of data common to all the terminals(e.g., DCI 0/1A). Terminal 200 (PDCCH receiving section 207) performsblind decoding for UE-SSs in three DCI formats; DCI containingallocation control information for the allocation of data common to allthe terminals (for example, DCI 0/1A), and DCI containing allocationcontrol information dependent on the transmission mode configured forterminal 200 (for example, uplink (DCI 0A, 0B and 4), and downlink (DCI1, 1B, 1D, 2, 2A, 2B and 2C)).

For example, when base station 100 configures only CRSs in an R-PDCCHregion as in the case of configuration method 1 in Embodiment 2, a casewill be described where base station 100 configures search spaces sothat the number of DCI allocation region candidates (number of blinddecoding region candidates) N_(B) in the R-PDCCH region is equal to orabove the number of DCI allocation region candidates (number of blinddecoding region candidates) N_(A) in the PDCCH region (N_(A)≦N_(B)).

In this case, as shown in FIG. 12A, in a PDCCH region for a certainterminal, search space configuration section 103 configures C-SSscorresponding to two DCI formats (12 candidates (=6 candidates×2 types))and configures UE-SSs corresponding to one DCI format (16 candidates).Furthermore, as shown in FIG. 12B, in an R-PDCCH region for a certainterminal, search space configuration section 103 configures UE-SSscorresponding to two DCI formats (32 candidates (=16 candidates×2types)). Therefore, in FIG. 12A and FIG. 12B, the number of DCIallocation region candidates (number of blind decoding regioncandidates) N_(B) (=32) in the R-PDCCH region is greater than the numberof DCI allocation region candidates (number of blind decoding regioncandidates) N_(A) (=28) in the PDCCH region.

In contrast, PDCCH receiving section 207 of terminal 200 blind-decodesDCI for C-SSs and UE-SSs shown in FIG. 12A and FIG. 12B according tosearch space region information inputted from configuration informationreceiving section 206. To be more specific, PDCCH receiving section 207performs 12 (=6 candidates×2 types) blind decoding operations for C-SSsin the PDCCH region shown in FIG. 12A and performs 16 (=16 candidates×1types) blind decoding operations for UE-SSs. Thus, the number of blinddecoding operations in the PDCCH region shown in FIG. 12A is 28.

Furthermore, PDCCH receiving section 207 blind-decodes UE-SSscorresponding to two DCI formats (16 candidates) in the R-PDCCH regionshown in FIG. 12B.

That is, in FIG. 12A and FIG. 12B, the number of blind decodingoperations (36 operations) in the R-PDCCH region is greater than thenumber of blind decoding operations (24 operations) in the PDCCH region.

That is, terminal 200 is more likely to be able to receive DCI forterminal 200 in an R-PDCCH region where other cell interference is morelikely to be reduced. Thus, when DCI for a terminal connected to a basestation is mapped onto a PDCCH region and an R-PDCCH region, the presentembodiment further reduces the possibility of receiving other cellinterference, and can thereby secure desired receiving quality in theterminal.

Furthermore, according to the present embodiment, the base stationconfigures search spaces for each DCI format. The base station canconfigure search spaces for each DCI format in an appropriatetransmission region of the PDCCH region or R-PDCCH region depending onfeatures of each DCI format (e.g., usage frequency, receiving quality ata location where usage is anticipated). Thus, the terminal can receiveDCI of each DCI format with desired receiving quality.

To be more specific, since the number of information bits of DCI 0/1A issmaller than the number of information bits of other DCI formatsdependent on the transmission mode, if the same number of CCEs isconfigured, DCI 0/1A allows transmission with a lower coding rate thanDCI 1, 1B, 1D, 2, 2A, 2B and 2C, 0A, 0B and 4. That is, when DCI 0/1A isconfigured by focusing on the fact that DCI 0/1A is a format less likelyto be influenced by a propagation path situation than other DCI formats,the base station may configure DCI 0/1A in the PDCCH region, andconfigure DCI 0A, 0B and 4 dependent on the uplink transmission mode andDCI 1, 1B, 1D, 2, 2A, 2B and 2C dependent on the downlink transmissionmode in the R-PDCCH region. That is, the base station configures acommon DCI format with the small amount of information in the PDCCHregion. This makes it possible to alleviate the tight condition of thePDCCH region (resources dependent on the uplink transmission mode areallocated to the R-PDCCH region), reduce influences of interference (DCIless likely to be influenced by a propagation path situation isallocated to the PDCCH region), and thereby achieve their respectiverequired receiving qualities.

The present embodiment is applicable not only to above-describedconfiguration method 1 but also to Embodiment 1 and configurationmethods 2 and 3 of Embodiment 2. For example, as in the case ofEmbodiment 1, applying the present embodiment to these embodiments canprevent throughput deterioration caused by the tight condition of aPDCCH region or R-PDCCH region in the entire system. Moreover, eachterminal is allowed to receive DCI using resources (PDCCH transmissionregion) suitable for a situation of each terminal and it is possible tosecure desired receiving quality for each terminal.

Furthermore, in the present embodiment, the base station and theterminal may also store allocation patterns (e.g., FIG. 13) of DCIallocation region candidates (blind decoding region candidates) in therespective search spaces of the PDCCH region and R-PDCCH region. Forexample, in FIG. 13, “∘” indicates that a search space for thecorresponding DCI format is configured for an allocation regioncorresponding to “∘,” and “−” indicates that a search space for thecorresponding DCI format is configured for allocation regioncorresponding to “−.” For example, in the case of search spaces ofpattern number 4 shown in FIG. 13, a C-SS corresponding to two DCIformats (DCI 1C and DCI 0/1A) and a UE-SS corresponding to one DCIformat (one of DCI 1, 1B, 1D, 2, 2A, 2B and 2C) are configured in thePDCCH region, and a UE-SS corresponding to two DCI formats (DCI 0/1A andDCI 0A, 0B and 4) is configured in the R-PDCCH region. The same appliesto search spaces with other pattern numbers shown in FIG. 13. Forexample, the base station may report pattern numbers shown in FIG. 13 toeach terminal depending on the situation (moving speed, position or thelike) of each terminal. Alternatively, the base station may select asearch space that allows the blocking probability to be reduced everytime (e.g., for each subframe) and report the selected pattern number tothe terminal.

Furthermore, the present embodiment has described configuration method 1of Embodiment 2 as an example. However, as in the case of configurationmethod 2 or 3 of Embodiment 2, the present embodiment may also beapplied to a case where base station 100 configures only DM-RSs in anR-PDCCH region. That is, the present embodiment may be applied to a casewhere search spaces are configured so that the number of DCI allocationregion candidates (number of blind decoding region candidates) N_(D) inan R-PDCCH region is equal to or above the number of DCI allocationregion candidates (number of blind decoding region candidates) N_(C) ina PDCCH region (N_(C)≦N_(D)).

Embodiment 4

A case has been described in Embodiment 3 where the base stationconfigures a transmission region (PDCCH region or R-PDCCH region)depending on features of each DCI format. The present embodiment willdescribe a specific example of the above configuration processing in thebase station according to Embodiment 3.

Since basic configurations of a base station and a terminal according tothe present embodiment are common to those in Embodiment 1, theseconfigurations will be described using FIG. 6 and FIG. 8.

Allocating section 106 of base station 100 disposes DCI containingallocation control information for common channels (e.g., DCI 1C and 1A)and DCI containing allocation control information for the allocation ofdata common to all the terminals (e.g., DCI 0/1A) in a PDCCH region, anddisposes DCI containing allocation control information dependent on thetransmission mode (e.g., uplink (DCI 0A, 0B and 4), downlink (DCI 1, 1B,1D, 2, 2A, 2B and 2C)) configured for terminal 200 in an R-PDCCH region.

That is, allocating section 106 allocates allocation control informationin a format common to all the terminals to search spaces configured inthe PDCCH region and allocates allocation control information in aformat corresponding to a transmission mode configured for terminal 200to search spaces configured in the R-PDCCH region.

Operation of search space configuration section 103 may be similar tothose of Embodiments 1 to 3.

In contrast, PDCCH receiving section 207 of terminal 200 blind-decodesDCI for C-SSs and UE-SSs according to search space region informationinputted from configuration information receiving section 206.

To be more specific, PDCCH receiving section 207 performs blind decodingfor C-SSs in the PDCCH region in DCI 1C and DCI 0/1A formats andperforms blind decoding for UE-SSs in a DCI format (DCI 0/1A) usedcommonly in plurality of transmission modes. As the number of blinddecoding operations in the PDCCH region, 12 (=6 candidates×2 types (DCI1C and DCI 0/1A, DCI formats 0 and 1A are blind-decoded as one type)blind decoding operations are performed for C-SSs and 16 (=16candidates×1 type) blind decoding operations are performed for UE-SSs.Thus, the number of blind decoding operations in the PDCCH region is 28.

Furthermore, PDCCH receiving section 207 performs blind decodingoperations in a DCI format determined dependently of the transmissionmode in the R-PDCCH region. In the R-PDCCH region, PDCCH receivingsection 207 blind-decodes a DCI format dependent on the downlinktransmission mode and a DCI format dependent on the uplink transmissionmode. When a transmission mode supporting only DCI 0 is selected incontrol information for an uplink, PDCCH receiving section 207blind-decodes DCI 0 in both the PDCCH region and the R-PDCCH region.

When PDCCH receiving section 207 blind-decodes 2 DCI formats in theR-PDCCH region, if 16 candidates are blind-decoded in the respective DCIformats, the number of blind decoding operations in the R-PDCCH regionis doubled to 32.

When terminal 200 configures the number of blind decoding operations asdescribed above, the number of blind decoding operations (32 operations)in the R-PDCCH region is greater than the number of blind decodingoperations (28 operations) in the PDCCH region.

That is, terminal 200 is more likely to receive DCI for terminal 200 inthe R-PDCCH region where other cell interference is more likely to bereduced.

Thus, when DCI for a terminal connected to a base station is mapped to aPDCCH region and an R-PDCCH region, the present embodiment can furtherreduce the possibility of receiving other cell interference. This allowsdesired receiving quality in the terminal to be secured.

Furthermore, according to the present embodiment, the base stationconfigures search spaces for each DCI format. By so doing, the basestation can configure search spaces for each DCI format in anappropriate transmission region among the PDCCH region and the R-PDCCHregion depending on features of each DCI format (e.g., usage frequencyand receiving quality at a location where usage is anticipated).

On a downlink, a DCI format commonly used among a plurality oftransmission modes is used mainly when receiving quality drasticallydeteriorates or when a change in the transmission mode cannot befollowed or the like. Therefore, the usage frequency of a DCI formatdependent on the transmission mode is considered to be higher than theusage frequency of a DCI format commonly used among a plurality oftransmission modes regardless of the transmission mode. As in the caseof the present embodiment, the terminal disposes a DCI format with ahigh usage frequency in the R-PDCCH region, and the R-PDCCH region isthereby mainly used. By so doing, the base station can more easilycontrol interference with other cells caused by the R-PDCCH in thefrequency-domain. Furthermore, the terminal can receive DCI transmittedwith interference controlled with desired receiving quality.

Furthermore, the number of information bits of DCI 0/1A is less than thenumber of information bits of other DCI dependent on the transmissionmode. For this reason, when the same number of CCEs is configured, thebase station can transmit data at a lower coding rate in DCI 0/1A thanin DCI 1, 1B, 1D, 2, 2A, 2B and 2C, 0A, 0B and 4. That is, when DCI 0/1Ais configured by focusing on the fact that DCI 0/1A is a format lesslikely to be influenced by a propagation path situation than other DCIformats, the base station configures DCI 0/1A in the PDCCH region andconfigures DCI 1, 1B, 1D, 2, 2A, 2B and 2C, 0A, 0B and 4 dependent onthe transmission mode in the R-PDCCH region. This makes it possible toreduce the tight condition of the PDCCH region (resources dependent onthe transmission mode are allocated to the R-PDCCH region), reduceinfluences of interference (DCI less likely to be influenced by thepropagation path situation is allocated to the PDCCH region), andthereby achieve their respective required receiving qualities.

Furthermore, when DCI 0/1A is configured in the PDCCH region and searchspaces in the R-PDCCH region are thereby changed, even if the terminalfails to receive a control signal for reporting a change in the searchspaces and a difference in recognition of the search spaces is producedbetween the terminal and the base station, the terminal can receive DCI0/1A transmitted in the PDCCH region and continue communication.Furthermore, even for a period during which search spaces are changed inthe R-PDCCH region, the terminal can receive DCI 0/1A transmitted to thePDCCH region and continue communication.

A case has been described in the present embodiment where when atransmission mode supporting only DCI 0 is selected in uplink controlinformation, the terminal blind-decodes DCI 0 in both the PDCCH regionand the R-PDCCH region. However, the terminal may also blind-decode DCI0 only in the PDCCH region. By so doing, it is possible to reduce thenumber of decoding operations on the uplink control information.

Embodiment 5

A case has been described in Embodiment 3 where attention is focused onwhether a DCI format is dependent on the transmission mode or not as anexample of configuring search spaces for each DCI format. In contrast,the present embodiment will describe a case where attention is focusedon DCI format family 1 and DCI format family 2 as an example ofconfiguring search spaces for each DCI format.

Since basic configurations of a base station and a terminal according tothe present embodiment are common to those in Embodiment 1, theseconfigurations will be described using FIG. 6 and FIG. 8. In the presentembodiment, operation of search space configuration section 103 of basestation 100 shown in FIG. 6 is different from that of Embodiment 1, FIG.14 illustrates an internal configuration of search space configurationsection 103 according to the present embodiment.

As shown in FIG. 14, search space configuration section 103 according tothe present embodiment has a configuration including transmission modedecision section 141 and configuration section 142.

Transmission mode decision section 141 decides a transmission modeconfigured for terminal 200 using configuration information inputtedfrom configuration section 101. To be more specific, transmission modedecision section 141 decides using the configuration information whetherthe downlink transmission mode configured for terminal 200 is atransmission mode including DCI format family 1 or a transmission modeincluding DCI format family 2. Transmission mode decision section 141outputs the decided transmission mode to configuration section 142.

Configuration section 142 configures common search spaces (C-SSs) andspecific search spaces (UE-SSs) based on a DCI transmission regionindicated by the configuration information inputted from configurationsection 101 (only PDCCH region or R-PDCCH region, or both PDCCH regionand R-PDCCH region) and the transmission mode inputted from transmissionmode decision section 141.

For example, a case will be described where both a PDCCH region and anR-PDCCH region are configured as DCI transmission regions. Upon decidingthat a transmission mode including DCI format family 1 is configured asthe DCI format dependent on the transmission mode among downlinktransmission modes, configuration section 142 configures DCI allocationregion candidates (blind decoding region candidates) of specific searchspaces (UE-SSs) in the PDCCH region. On the other hand, upon decidingthat a transmission mode including DCI format family 2 is configured asthe downlink transmission mode, configuration section 142 configures DCIallocation region candidates (blind decoding region candidates) ofspecific search spaces (UE-SSs) in the PDCCH region and the R-PDCCHregion, and configuration section 142 further configures DCI allocationregion candidates in a DCI format not dependent on the transmission modein the PDCCH region, and configures DCI allocation region candidates ina DCI format dependent on the transmission mode in the R-PDCCH region.

The present embodiment does not impose any restrictions on whether C-SSsshould be configured in the PDCCH region or R-PDCCH region. However,configuration section 142 may configure C-SSs in the PDCCH region inconsideration that a load balance should be kept between the PDCCHregion and R-PDCCH region or that an error resistant DCI format shouldbe disposed in C-SSs.

Furthermore, configuration section 142 may configure a DCI formatsubjected to Temporary C-RNTI which is used only for RACH processing inthe PDCCH region. This is because the configuration such as allocationof the R-PDCCH region or the like is not performed at timing of RACHprocessing.

Allocating section 106 allocates DCI containing allocation controlinformation to CCEs in C-SSs or UE-SSs using search space informationfrom search space configuration section 103.

To be more specific, allocating section 106 allocates DCI correspondingto DCI format family 2 (e.g., DCI 2, 2A, 2B and 2C) of allocationcontrol information dependent on the transmission mode configured forthe terminal to CCEs in UE-SSs in the R-PDCCH region.

Furthermore, allocating section 106 allocates DCI corresponding to DCIformat family 1 (e.g., DCI 1, 1B and 1D) of allocation controlinformation dependent on the transmission mode configured for theterminal to CCEs in UE-SSs in the PDCCH region.

Furthermore, allocating section 106 allocates DCI containing allocationcontrol information for common channels (e.g., DCI 1C and 1A) to CCEs inC-SSs. Furthermore, allocating section 106 allocates DCI containingallocation control information for the allocation of data used in aplurality of transmission modes (e.g., DCI 0/1A) to CCEs in C-SSs orCCEs in UE-SSs.

In contrast, in terminal 200, PDCCH receiving section 207 blind-decodesDCI containing allocation control information for common channels (e.g.,DCI 1C and 1A) and DCI containing allocation control information for theallocation of data used in plurality of transmission modes (e.g., DCI0/1A) for C-SSs indicated by a search space region indicated by thesearch space region information inputted from configuration informationreceiving section 206. Furthermore, PDCCH receiving section 207blind-decodes DCI containing allocation control information for theallocation of data used in a plurality of transmission modes (e.g., DCI0/1A) and DCI containing allocation control information dependent on thetransmission mode configured for terminal 200 (e.g., uplink (DCI 0A, 0Band 4), downlink (DCI 1, 1B and 1D, 2, 2A, 2B and 2C)) for UE-SSsindicated by a search space region indicated by the search space regioninformation inputted from configuration information receiving section206.

That is, for C-SSs, terminal 200 performs blind decoding on two DCIformats (DCI 1C and 1A and DCI 0/1A). On the other hand, for UE-SSs,terminal 200 performs blind decoding on three DCI formats (DCI dependenton the uplink transmission mode (DCI 0A, 0B and 4), DCI dependent on thedownlink transmission mode (DCI 1, 1B, 1D, 2, 2A, 2B and 2C) and DCI0/1A). However, when the uplink transmission mode supports only DCI0,terminal 200 performs blind decoding on two DCI formats.

When the search space region information is predetermined by a cell IDor the like or even when the search space region information is decidednot to be updated, if the transmission mode configured for terminal 200includes DCI format family 2, PDCCH receiving section 207 may performblind decoding for UE-SSs limited to the R-PDCCH region. Furthermore,when the transmission mode configured for terminal 200 includes DCIformat family 1, PDCCH receiving section 207 may perform blind decodingfor UE-SSs limited to the PDCCH region. Furthermore, PDCCH receivingsection 207 may perform blind decoding for C-SSs limited to the PDCCHregion.

Even when the search space region information is not updated (whenconfiguration information from an upper layer is not updatedsimultaneously with update of the transmission mode), if a transmissionmode including DCI format family 1 is configured, PDCCH receivingsection 207 may perform blind decoding for UE-SSs limited to the PDCCHregion.

For example, a case will be described where a transmission modeincluding DCI format family 2 is configured and both a PDCCH region andan R-PDCCH region are configured in the search space region information.That is, terminal 200 performs blind decoding for UE-SSs in both thePDCCH region and the R-PDCCH region. In this case, when the search spaceregion information is not updated and the transmission mode is changedto one including DCI format family 1, PDCCH receiving section 207performs blind decoding operations for UE-SSs limited to the PDCCHregion. That is, although the search space region information isconfigured such that the R-PDCCH region is included as a search space,if a transmission mode including DCI format family 1 is configured,PDCCH receiving section 207 blind-decodes only the PDCCH region as asearch space.

Furthermore, when the transmission mode is changed to a transmissionmode including DCI format family 2 again, terminal 200 may use theconfiguration of the newest R-PDCCH region in the reported configurationinformation. By so doing, terminal 200 need not configure an R-PDCCHregion every time the transmission mode is changed, and can therebyreduce control signals for configuring the R-PDCCH region.

Here, among the downlink DCI formats (DCI 1A, 1, 1B and 1D, 2, 2A, 2Band 2C), DCI format family 2 (DCI 2, 2A, 2B and 2C) is controlinformation to allocate spatial multiplexing MIMO transmissioncorresponding to a plurality of layers. Thus, DCI format family 2 hasmore information bits than other downlink DCI formats. Furthermore, aterminal to which DCI format family 2 is applied is a terminalsupporting spatial multiplexing MIMO transmission adaptable to aplurality of layers. Thus, DM-RSs are more likely to be indicated to beused as reference signals for the terminal.

Furthermore, as shown in FIG. 15, when reference signals used are CRSs,a frequency diversity effect is obtained in the PDCCH region in commonamong terminals. For this reason, when reference signals used are CRSs,there are assumed to be no differences in reception performance (e.g.,reception SINR) depending on the location where search spaces areconfigured. On the other hand, as shown in FIG. 15, when referencesignals used are DM-RSs, the location where search spaces are configuredis an R-PDCCH region. When reference signals used are DM-RSs, it ispossible to obtain an effect that a frequency resource having goodreception performance is specifically selected for the correspondingterminal (frequency selectivity effect) and a precoding gain throughbeam forming specifically adjusted for the terminal. For this reason,when reference signals used are DM-RSs, improvement in receptionperformance (reception SINR) can be expected.

That is, when reference signals used are DM-RSs, it is possible tofurther improve reception performance (reception SINR) and reduce thenumber of CCEs required compared to a case where reference signals usedare CRSs. That is, the greater the number of information bits of controlinformation used, the greater the effect of a reduction in the number ofCCEs used becomes.

Thus, when a transmission mode including DCI format family 2 with manyinformation bits is configured, base station 100 (search spaceconfiguration section 103) configures more UE-SSs in the R-PDCCH regionand allocating section 106 allocates DCI to CCEs in UE-SSs configured inthe R-PDCCH region. By so doing, DM-RSs are more likely to be configuredin the transmission mode including DCI format family 2. It is therebypossible to reduce the number of CCEs used in the transmission modeincluding DCI format family 2 and reduce the resource amount used in thePDCCH region and R-PDCCH region as a whole. In other words, when theresource amount used is assumed to be constant, more terminals can beaccommodated.

Furthermore, for example, in a HetNet environment, since an arrangementof resources in the PDCCH region is defined beforehand by a cell ID orthe like, it is difficult to avoid interference. In contrast, in theHetNet environment, since an arrangement of resources in the R-PDCCHregion can be scheduled, it is possible to avoid interference. Thus,interference among cells is more likely to be reduced throughinterference control or the like in an R-PDCCH region than in a PDCCHregion. That is, the reception performance (reception SINR) is morelikely to be improved in an R-PDCCH region than in a PDCCH region.

On the other hand, when the same number of CCEs is configured in adownlink DCI format, as shown in FIG. 16, a DCI format with moreinformation bits has a larger Mary modulation value and a higher codingrate. That is, as shown in FIG. 16, a DCI format with more informationbits has lower error resistance.

Thus, when a transmission mode including DCI format family 2 with moreinformation bits is configured, base station 100 (search spaceconfiguration section 103) configures more UE-SSs in the R-PDCCH regionand allocating section 106 allocates DCI to CCEs in UE-SSs configured inthe R-PDCCH region. By so doing, it is possible to transmit a DCI formatof weak error resistance (DCI format family 2) in the R-PDCCH regionhaving better reception performance than in the PDCCH region. That is,search space configuration section 103 can configure search spacescapable of securing desired receiving quality for the DCI format of lowerror resistance (DCI format family 2).

In contrast, when the same number of CCEs is configured in a downlinkDCI format, as shown in FIG. 16, a DCI format having fewer informationbits has a smaller M-ary modulation value and a lower coding rate. Thatis, as shown in FIG. 16, a DCI format having fewer information bits hashigher error resistance.

Among downlink DCI formats (DCI 1A, 1, 1B and 1D, 2, 2A, 2B and 2C), DCIformat family 1 (DCI 1, 1B and 1D) is control information for allocatingtransmission corresponding to a single layer. For this reason, DCIformat family 1 has fewer information bits than DCI format family 2.

Therefore, when a transmission mode including DCI format family 1 havingfewer information bits is configured, base station 100 (search spaceconfiguration section 103) configures more UE-SSs in the PDCCH regionand allocating section 106 allocates DCI to CCEs in UE-SSs configured inthe PDCCH region. By so doing, a DCI format having high error resistance(DCI format family 1) is transmitted in the PDCCH region of poorerreception performance than the R-PDCCH region. That is, even whentransmitting a DCI format having high error resistance in a transmissionregion of poor reception performance, search space configuration section103 can configure search spaces capable of securing desired receivingquality.

Furthermore, by configuring a DCI format having low error resistance(DCI format family 2) in the R-PDCCH region and configuring a DCI formathaving high error resistance (DCI format family 1) in the PDCCH region,base station 100 can keep a load balance relating to resourceutilization between the PDCCH region and R-PDCCH region.

In this way, base station 100 determines in which region (PDCCH regionor R-PDCCH region) DCI should be transmitted in consideration of thesituation of each terminal (e.g., position of the terminal, magnitude ofother cell interference, traffic situation (e.g., “PDCCH region isbecoming tight as the number of communicating terminals increases”) orthe like). To be more specific, the base station configures a specificDCI format having the large amount of information (e.g., DCI formatfamily 2) in the R-PDCCH region. Furthermore, the base stationconfigures a specific DCI format having the small amount of information(DCI format family 1) in the PDCCH region.

Thus, it is possible to prevent throughput deterioration caused by thetight condition of the PDCCH region or R-PDCCH region in the entiresystem. Furthermore, each terminal can receive DCI using resources(PDCCH transmission region) suitable for a situation of each terminaland secure desired receiving quality in the terminal.

Therefore, according to the present embodiment, even when DCI for aterminal connected to a base station is allocated to a PDCCH region andan R-PDCCH region, it is possible to secure desired receiving quality inthe terminal without causing system throughput to deteriorate.

The present embodiment has described a DCI format dependent on adownlink transmission mode in particular. However, the presentembodiment may also apply operation similar to that of DCI format family2 to DCI format 4 (DCI 4) in which spatial multiplexing MIMOtransmission is allocated in a DCI format dependent on an uplinktransmission mode. Furthermore, the above-described operation wherebyDCI format 4 is configured in an R-PDCCH region may be limited to only acase where DCI format family 2 is used as a downlink transmission mode.

Embodiment 6

A case has been described in Embodiments 1 to 3 where the base stationconfigures search spaces based on the relationship of the number of DCIallocation region candidates (number of blind decoding regioncandidates) between a PDCCH region and an R-PDCCH region in theterminal. In contrast, in the present embodiment, the base stationconfigures search spaces based on the relationship in a search spacesize (the number of CCEs constituting a search space) between a PDCCHregion and an R-PDCCH region.

Since basic configurations of a base station and a terminal according tothe present embodiment are common to those in Embodiment 1, theseconfigurations will be described using FIG. 6 and FIG. 8.

In the following description, as in the case of Embodiment 1, 6 DCIallocation region candidates (blind decoding region candidates) in total(total 32 CCEs); 4 candidates (16 CCEs) for CCE aggregation level 4 and2 candidates (16 CCEs) for CCE aggregation level 8, are configured asC-SSs. Furthermore, 16 DCI allocation region candidates (blind decodingregion candidates) (total 42 CCEs) in total; 6 candidates (6 CCEs), 6candidates (12 CCEs), 2 candidates (8 CCEs) and 2 candidates (16 CCEs)for CCE aggregation levels 1, 2, 4 and 8 respectively are configured asUE-SSs. That is, search spaces (C-SS and UE-SS) composed of 74 CCEs intotal are configured for each terminal.

Search space configuration section 103 of base station 100 according tothe present embodiment configures search spaces for each terminal sothat the ratio of the search space size (that is, the number of CCEsconstituting a search space) in an R-PDCCH region to the search spacesize in a PDCCH region is greater in a terminal using DM-RSs in theR-PDCCH region than in a terminal using CRSs in the R-PDCCH region.

For example, as shown in FIG. 17A, for a certain terminal using CRSs inthe R-PDCCH region, search space configuration section 103 configuresC-SSs of 32 CCEs in total; candidates (16 CCEs) of CCE aggregation level8 and 4 candidates (16 CCEs) of CCE aggregation level 4, and configuresUE-SSs of 24 CCEs in total; 2 candidates (16 CCEs) of CCE aggregationlevel 8 and 2 candidates (8 CCEs) of CCE aggregation level 4 in thePDCCH region. Furthermore, as shown in FIG. 17B, for a certain terminalusing CRSs in the R-PDCCH region, search space configuration section 103configures UE-SSs of 18 CCEs in total; 6 candidates (12 CCEs) of CCEaggregation level 2 and 6 candidates (6 CCEs) of CCE aggregation level 1in the R-PDCCH region.

On the other hand, as shown in FIG. 17A, for a certain terminal usingDM-RSs in the R-PDCCH region, search space configuration section 103configures UE-SSs of 18 CCEs in total; 6 candidates (12 CCEs) of CCEaggregation level 2 and 6 candidates (6 CCEs) of CCE aggregation level 1in the PDCCH region. Furthermore, as shown in FIG. 17B, for a certainterminal using DM-RSs in the R-PDCCH region, search space configurationsection 103 configures C-SSs of 32 CCEs in total; candidates (16 CCEs)of CCE aggregation level 8 and 4 candidates (16 CCEs) of CCE aggregationlevel 4 in the R-PDCCH region, and configures UE-SSs of 24 CCEs intotal; 2 candidates (16 CCEs) of CCE aggregation level 8 and 2candidates (8 CCEs) of CCE aggregation level 4.

That is, search space configuration section 103 configures search spaces(C-SS and UE-SS) so that the ratio of the search space size (56 CCEs) inthe R-PDCCH region to the search space size (18 CCEs) in the PDCCHregion for terminal 200 using DM-RSs in the R-PDCCH region (56/18) isequal to or greater than the ratio of the search space size (18 CCEs) inthe R-PDCCH region to the search space size (56 CCEs) in the PDCCHregion in terminal 200 using CRSs in the R-PDCCH region (18/56).

The configuration of search spaces is not limited to the above-describedone, but as shown in configuration method 3 of Embodiment 2, for acertain terminal using DM-RSs in the R-PDCCH region, C-SSs of 32 CCEs intotal may be configured in the PDCCH region; 2 candidates (16 CCEs) ofCCE aggregation level 8 and 4 candidates (16 CCEs) of CCE aggregationlevel 4, and UE-SSs of 32 CCEs in total may be configured in the R-PDCCHregion; 2 candidates (16 CCEs) of CCE aggregation level 8, 2 candidates(8 CCEs) of CCE aggregation level 4, 6 candidates (12 CCEs) of CCEaggregation level 2 and 6 candidates (6 CCEs) of CCE aggregationlevel 1. In this case, search spaces (C-SS and UE-SS) are alsoconfigured so that the ratio of the search space size (32 CCEs) in theR-PDCCH region to the search space size (32 CCEs) in the PDCCH regionfor terminal 200 using DM-RSs in the R-PDCCH region (32/32) is equal toor above the ratio of the search space size (18 CCEs) in the R-PDCCHregion to the search space size (56 CCEs) in the PDCCH region forterminal 200 using CRSs in the R-PDCCH region (18/56).

That is, as shown in FIG. 17B, in the R-PDCCH region, search spaceconfiguration section 103 can configure search spaces using more CCEsfor a terminal using DM-RSs in the R-PDCCH region (terminal havinggreater other cell interference) than a terminal using CRSs in theR-PDCCH region.

Furthermore, as described above, other cell interference is more likelyto be reduced in the R-PDCCH region than in the PDCCH region throughinterference control or the like.

Thus, for a terminal using DM-RSs, DCI is more likely to be received inthe R-PDCCH region that occupies a greater part (more CCEs) of theconfigured search space. Therefore, the terminal using DM-RSs obtains aPDCCH received signal power improving effect using DM-RSs (see FIG. 3)while suppressing influences from other cell interference in the R-PDCCHregion, and can thereby secure desired receiving quality in theterminal.

Here, for example, for a terminal using CRSs in the R-PDCCH region, basestation 100 allocates data in search spaces using relatively more CCEsin the PDCCH region. On the other hand, for a terminal using DM-RSs inthe R-PDCCH region, base station 100 allocates data in search spacesusing relatively more CCEs in the R-PDCCH region. This makes it possibleto maintain a load balance in a PDCCH to which control information foreach terminal is allocated. That is, it is possible to avoid PDCCHs frombeing biased to a specific region preventing PDCCHs from beingtransmitted within a limited region. This prevents throughputdeterioration caused by the tight condition of the PDCCH region andR-PDCCH region.

Furthermore, base station 100 may use, for example, a PDCCH region or anR-PDCCH region using CRSs (reference signals capable of obtaining afrequency diversity effect) for DCI for a terminal that is moving fast(with a violent fluctuation in the received signal level). On the otherhand, base station 100 may use an R-PDCCH region using DM-RSs (referencesignals that allow received signal power to be improved) for DCI for aterminal located near a cell edge (with a low received signal level).This makes it possible to realize search space allocation capable ofsecuring desired receiving quality regardless of the moving speed orlocation area or the like of a terminal. This allows each terminal toreceive DCI using resources suitable for a situation of each terminal(PDCCH transmission region) and secure desired receiving quality of theterminal.

Thus, according to the present embodiment, even when mapping DCI for aterminal connected to a base station onto the PDCCH region and R-PDCCHregion, it is possible to secure desired receiving quality in a terminalwithout causing system throughput to deteriorate.

A case has been described in the present embodiment where a terminalusing CRSs and a terminal using DM-RSs coexist in the R-PDCCH region. Incontrast, the present embodiment is also applicable to a case wherereference signals used for each terminal in an R-PDCCH region arefixedly configured. For example, base station 100 may configure searchspaces so that the search space size in the R-PDCCH region is equal toor greater than the search space size in the PDCCH region as in the caseof configuration method 1 in Embodiment 2 (when DCI allocation regioncandidates are used).

For example, as shown in FIG. 18, search space configuration section 103configures UE-SSs of 18 CCEs in total in a PDCCH region for a certainterminal using only CRSs in an R-PDCCH region and configures C-SSs of 32CCEs in total and UE-SSs of 24 CCEs in total in the R-PDCCH region. Thatis, in FIG. 18, the search space size in the R-PDCCH region (56 CCEs) isgreater than the search space size in the PDCCH region (18 CCEs).

Thus, by increasing the ratio of the search space configured in theR-PDCCH region among search spaces configured in terminal 200, terminal200 is more likely to receive DCI in the R-PDCCH region that occupies agreater part (more CCEs) of the configured search space. This allowsterminal 200 to suppress influences of other cell interference andsecure desired receiving quality.

Furthermore, the present embodiment is not limited to Embodiment 1 andconfiguration method 1 of Embodiment 2, but may configure search spacesbased on a search space size in the same way as configuration methods 2and 3 of Embodiment 2 and Embodiment 3 (when the number of blinddecoding operations is used).

Embodiment 7

The present embodiment will describe a case where a TDM+FDMconfiguration is applied to an R-PDCCH region. However, suppose whetherthe R-PDCCH region has an FDM configuration or a TDM+FDM configurationis configured beforehand.

Since basic configurations of a base station and a terminal according tothe present embodiment are common to those in Embodiment 1, theseconfigurations will be described using FIG. 6 and FIG. 8. In the presentembodiment, operation of search space configuration section 103 of basestation 100 shown in FIG. 6 is different from that of Embodiment 1. FIG.19 shows an internal configuration of search space configuration section103 according to the present embodiment.

In base station 100 according to the present embodiment, search spaceconfiguration section 103 adopts a configuration including transmissionformat decision section 151, data allocation decision section 152 andconfiguration section 153. Transmission format decision section 151decides a DCI format configured for terminal 200 using configurationinformation inputted from configuration section 101. To be morespecific, transmission format decision section 151 decides whether ornot the transmission format configured for terminal 200 is a common DCIformat and whether or not the transmission format is DCI 1Acorresponding to downlink allocation control information subject toconsecutive band allocation. Transmission format decision section 151outputs the decision result to configuration section 153.

Data allocation decision section 152 decides whether or not transmissiondata is allocated to terminal 200. Data allocation decision section 152outputs the decision result to configuration section 153.

Configuration section 153 configures common search spaces (C-SSs) andspecific search spaces (UE-SSs). For example, when both a PDCCH regionand an R-PDCCH region are configured as DCI transmission regions,configuration section 153 configures search spaces (C-SS and UE-SS)having the aforementioned plurality of DCI allocation region candidatesin the PDCCH region and R-PDCCH region.

Particularly, configuration section 153 configures search spaces fordisposing DCI 0 which is a common DCI format and which corresponds touplink allocation control information subject to consecutive bandallocation based on a DCI transmission region indicated by theconfiguration information inputted from configuration section 101 (onlyPDCCH region or R-PDCCH region, or both PDCCH region and R-PDCCHregion), the decision result inputted from transmission format decisionsection 151 (DCI 1A or not) and the decision result inputted from dataallocation decision section 152 (presence or absence of dataallocation).

To be more specific, as shown in FIG. 20, when DCI 1A is configured forterminal 200 (format 1A: Yes) or when there is data allocation (dataallocation: Yes), configuration section 153 configures search spaces(C-SS or UE-SS) for disposing DCI 0 in the PDCCH region. As describedabove, to suppress an increase in the number of blind decodingoperations, C-SSs are more likely to be indicated for DCI 0.

On the other hand, as shown in FIG. 20, when DCI 1A is not configuredfor terminal 200 (format 1A: No) and when there is no data allocation(data allocate: No), configuration section 153 configures search spaces(C-SS or UE-SS) for disposing DCI 0 in slot 1 (2nd slot) of the R-PDCCHregion. UE-SSs are more likely to be indicated for DCI 0 also inconsideration of a case where DM-RSs are configured in the R-PDCCHregion.

Allocating section 106 then allocates DCI containing allocation controlinformation to CCEs in C-SSs or UE-SSs using search space informationfrom search space configuration section 103. In particular, allocatingsection 106 allocates DCI corresponding to DCI 0 to a transmissionregion (PDCCH region or slot 1 (2nd slot) of the R-PDCCH region)configured by configuration section 153 of search space configurationsection 103.

In contrast, PDCCH receiving section 207 in terminal 200 blind-decodesDCI containing allocation control information for common channels (e.g.,DCI 1C and 1A) and DCI containing allocation control information for theallocation of data common to all the terminals (e.g., DCI 0/1A) forC-SSs indicated by a search space region indicated by the search spaceregion information inputted from configuration information receivingsection 206. Furthermore, PDCCH receiving section 207 blind-decodes DCIcontaining allocation control information for the allocation of datacommon to all the terminals (e.g., DCI 0/1A) and DCI containingallocation control information dependent on the transmission modeconfigured for terminal 200 (e.g., uplink (DCI 0A, 0B and 4), downlink(DCI 1, 1B and 1D, 2, 2A, 2B and 2C)) for UE-SSs indicated by the searchspace region indicated by the search space region information inputtedfrom configuration information receiving section 206. Particularly,PDCCH receiving section 207 specifies search spaces in which DCI 0 forterminal 200 is disposed based on the detection result of DCI 1A forterminal 200 and the presence or absence of data allocation for terminal200.

The search space configuration method according to the presentembodiment will be described more specifically below.

As described above, the following matters should be considered about anR-PDCCH region in a TDM+FDM configuration with respect to a relaystation (relay node).

(a) A DL grant is transmitted in slot 0 (1st slot) and a UL grant istransmitted in slot 1 (2nd slot).

(b) When a data signal (PDSCH) is indicated by an R-PDCCH, a PDSCH istransmitted using only slot 1 or both slot 0 and slot 1 (that is, datatransmission in only slot 0 is not possible).

The above-described matters about the R-PDCCH region with respect to therelay station are also more likely to be applicable to the R-PDCCHregion with respect to the terminal.

Thus, as shown in case 1 in FIG. 21, when DCI 1A is configured in thePDCCH region as a DL grant for terminal 200, search space configurationsection 103 configures a search space for disposing DCI 0 in the PDCCHregion.

Upon detecting DCI 1A (DL grant) for terminal 200 in the PDCCH region,PDCCH receiving section 207 decides that DCI 0 for terminal 200 isconfigured in the PDCCH region. PDCCH receiving section 207 then limitsblind decoding on DCI 0 to the PDCCH region.

Thus, as shown in case 1 in FIG. 21, since DCI 1A and DCI 0 are disposedtogether in the PDCCH region, base station 100 can allocate data in theR-PDCCH region. Alternatively, base station 100 can use the R-PDCCHregion as a PDSCH and an R-PDCCH for another apparatus (relay station orother terminal).

As shown in case 2 in FIG. 21, when DCI 1A for terminal 200 is notconfigured and data is allocated, search space configuration section 103configures a search space for disposing DCI 0 in the PDCCH region.

When no DCI 1A for terminal 200 is not detected in the PDCCH region,PDCCH receiving section 207 decides that DCI 0 directed to terminal 200is configured in slot 1 (2nd slot) of the PDCCH region or R-PDCCHregion. PDCCH receiving section 207 then limits blind decoding on DCI 0to slot 1 (2nd slot) of the PDCCH region or R-PDCCH region. For example,as a result of detecting downlink control information (DCI 1, 1B and 1D,2, 2A, 2B and 2C) dependent on the transmission mode in slot 0 (1stslot) of the R-PDCCH region, if data allocation is found, PDCCHreceiving section 207 decides that DCI 0 is configured in the PDCCHregion. PDCCH receiving section 207 then limits blind decoding on DCI 0to the PDCCH region.

Thus, as shown in case 2 in FIG. 21, base station 100 can allocate a DLgrant to slot 0 (1st slot) in the R-PDCCH region and allocate data toslot 1 (2nd slot) in the R-PDCCH region.

Furthermore, as shown in case 3 in FIG. 21, when no DCI 1A for terminal200 is configured and no data allocation is found, search spaceconfiguration section 103 configures a search space for disposing DCI 0in slot 1 (2nd slot) of the R-PDCCH region.

When DCI 1A for terminal 200 is not detected in the PDCCH region, anddata allocation is not found as a result of detecting downlink controlinformation dependent on the transmission mode for terminal 200 (DCI 1,1B and 1D, 2, 2A, 2B and 2C) in slot 0 (1st slot) of the R-PDCCH region,PDCCH receiving section 207 decides that DCI 0 is configured in slot 1(2nd slot) of the R-PDCCH region. PDCCH receiving section 207 thenlimits blind decoding on DCI 0 to slot 1 (2nd slot) of the R-PDCCHregion.

Thus, as shown in case 3 in FIG. 21, base station 100 disposes DCI 0 inthe R-PDCCH region, and can thereby reduce the resource amount (that is,overhead) used in the PDCCH region. Alternatively, base station 100 canuse the PDCCH region for other terminals.

When no search space region information is inputted from configurationinformation receiving section 206 (when base station 100 does nottransmit search space information), PDCCH receiving section 207 mayperform blind decoding in a plurality of DCI transmission regions whichmay be directed to terminal 200 without being aware of search spaces ofterminal 200.

In this case, PDCCH receiving section 207 may also decide blind decodinglocations of DCI 0 based on whether DCI 1A is detected in the PDCCHregion or not and whether data is allocated or not. Whether data isallocated or not is decided based on whether a DCI format of downlinkallocation control information dependent on the transmission mode isdetected in slot 0 (1st slot) of the R-PDCCH region or not.

That is, upon detecting DCI 1A in the PDCCH region, PDCCH receivingsection 207 decides that DCI 0 is also configured in the same searchspace and blind-decodes the PDCCH region. On the other hand, upon notdetecting DCI 1A (nor detecting DCI 0 in the PDCCH region), PDCCHreceiving section 207 decides that DCI 0 is disposed in slot 1 (2ndslot) of the R-PDCCH region.

When no data allocation is found as a result of detecting a DCI formatof downlink allocation control information dependent on the transmissionmode in slot 0 (1st slot) of the R-PDCCH region, PDCCH receiving section207 decides that DCI 0 is disposed in slot 1 (2nd slot) of the R-PDCCHregion. Furthermore, when data allocation is found (and DCI 0 is notdetected in the PDCCH region), PDCCH receiving section 207 may decidethat uplink allocation control information dependent on the transmissionmode (one of DCI 0A, 0B and 4) is disposed in slot 1 (2nd slot) of theR-PDCCH region and perform blind decoding.

In this way, according to the present embodiment, when the R-PDCCHregion has a TDM+FDM configuration, the base station disposes DCI 0 inthe PDCCH region or slot 1 (2nd slot) of the R-PDCCH region based on thepresence or absence of DCI 1A and the presence or absence of dataallocation. This allows the base station to improve flexibility ofscheduling and effectively use resources. Furthermore, since DCI 0 isalso disposed in slot 1 (2nd slot) of the R-PDCCH region, it is possibleto reduce resources used in the PDCCH region. Furthermore, DCI 0 is alsodisposed in slot 1 (2nd slot) of the R-PDCCH region, which increases theprobability that the number of necessary CCEs may be reduced asdescribed above, and it is possible to reduce resources used in thePDCCH region and R-PDCCH region as a whole. In other words, when theresource amount used is assumed to be constant, more terminals can beaccommodated.

Thus, according to the present embodiment, even when DCI for a terminalconnected to a base station is mapped onto a PDCCH region and an R-PDCCHregion, it is possible to secure desired receiving quality in theterminal without causing system throughput to deteriorate.

Each embodiment of the present invention has been describedhereinbefore.

In each embodiment above, the base station and terminal may storeallocation patterns (e.g., FIG. 22) of blind decoding region candidatesin respective search spaces of the PDCCH region and R-PDCCH region. Forexample, in the case of search spaces in pattern number 2 shown in FIG.22, C-SSs of 6 candidates in total; 2 candidates of CCE aggregationlevel 8 and 4 candidates of CCE aggregation level 4, and UE-SSs of 4candidates in total; 2 candidates of CCE aggregation level 8 and 2candidates of CCE aggregation level 4 are configured in the PDCCHregion, and UE-SSs of 12 candidates in total; 6 candidates of CCEaggregation level 2 and 6 candidates of CCE aggregation level 1 areconfigured in the R-PDCCH region. The same applies to search spaces withother pattern numbers shown in FIG. 22. For example, the base stationmay report pattern numbers shown in FIG. 22 to the terminal depending onthe situation (moving speed, position or the like) of each terminal.Alternatively, the base station may select a search space that allows ablocking probability to be reduced every time (e.g., subframe bysubframe) and report the selected pattern number to the terminal.

Furthermore, the present invention is not limited to the aboveembodiments, but may be implemented modified in various ways. Forexample, the present invention may be implemented by combining therespective embodiments as appropriate depending on the situation of eachterminal.

Furthermore, Cell-Radio Network Temporary Identifier (C-RNTI) may alsobe used for the terminal ID in the above embodiments.

Furthermore, the expression “DCI format common to all the terminals” inthe above embodiments may also be read as “DCI format independent of atransmission mode.”

Although a case has been described in the above embodiments where DCI0/1A is used as “DCI format common to all the terminals,” the presentinvention is not limited to this, but any format may be used if it canbe used regardless of a transmission mode.

Furthermore, a case has been described in the above embodiments whereDCI 0A, 0B, 1, 1B, 1D, 2 or 2A is used as DCI dependent on thetransmission mode. However, formats other than DCI 1, 1B, 1D, 2, 2A, 2B,2C, 0A, 0B and 4 may also be used as DCI dependent on the transmissionmode.

Furthermore, consecutive band allocation transmission may also beincluded as an uplink or downlink transmission mode. For a terminal inwhich this transmission mode is set, DCI dependent on the transmissionmode is DCI 0 (uplink) and DCI 1A (downlink) respectively. In this case,since the DCI format common to all the terminals and the formatdependent on the transmission mode are identical, blind decoding may beperformed for UE-SSs on one format for the uplink and downlinkrespectively. One format in total is used in the case of consecutiveband allocation for both the uplink and downlink. Thus, configuring DCI0/1A for DCI dependent on the transmission mode having a wider searchspace can prevent an increase in the blocking probability for a terminalto which PDCCH can be allocated using only DCI 0/1A because itspropagation path situation is originally poor.

Furthermore, CCEs described in the above embodiments are logicalresources and when CCEs are disposed in actual physical time/frequencyresources, CCEs are disposed distributed over the entire bandwidthwithin a component band. Furthermore, if only CCEs are divided based onthe unit of component band as logical resources, CCEs may be disposed inactual physical time/frequency resources distributed over the entiresystem band (that is, over all component bands).

Furthermore, the terminal may be called “UE” and the base station may becalled “Node B” or “BS (Base Station).” Furthermore, the terminal ID maybe called “ID” or “UE-ID.”

In the foregoing embodiments, the present invention is configured withhardware by way of example, but the invention may also be provided bysoftware in cooperation with hardware.

The functional blocks used in the description of the embodiments may betypically implemented as an LSI, an integrated circuit. They may beindividual chips, or some of or all of them may be integrated into asingle chip. “LSI” is used here, but “1C,” “system LSI,” “super LSI,” or“ultra LSI” may also be adopted depending on the degree of integration.

Alternatively, circuit integration may also be implemented using adedicated circuit or a general processor other than an LSI. After an LSIis manufactured, an FPGA (field programmable gate array) or areconfigurable processor which enables the reconfiguration of connectionand setting of circuit cells in an LSI may be used.

If integrated circuit technology appears to replace LSIs as a result ofthe advancement of semiconductor technology or other derivativetechnology, the functional blocks could be integrated using thistechnology. Biotechnology can also be applied.

The disclosure of the specification, the drawings, and the abstractincluded in Japanese Patent Application No. 2010-164307, filed on Jul.21, 2010, and Japanese Patent Application No. 2011-045088 filed on Mar.2, 2011, is incorporated herein by reference in its entirety.

INDUSTRIAL APPLICABILITY

The present invention is useful for a mobile communication system.

REFERENCE SIGNS LIST

-   100 base station-   101, 142, 153 configuration section-   102 control section-   103 search space configuration section-   104 PDCCH generating section-   105, 107, 109 coding/modulation section-   106 allocating section-   108 transmission weight configuration section-   110 multiplexing section-   111, 213 IFFT section-   112, 214 CP adding section-   113, 215 RF transmitting section-   114, 201 antenna-   115, 202 RF receiving section-   116, 203 CP removing section-   117, 204 FFT section-   118 extracting section-   119 IDFT section-   120 data receiving section-   121 ACK/NACK receiving section-   200 terminal-   205 demultiplexing section-   206 configuration information receiving section-   207 PDCCH receiving section-   208 PDSCH receiving section-   209, 210 modulation section-   211 DFT section-   212 mapping section-   141 transmission mode decision section-   151 transmission format decision section-   152 data allocation decision section

The invention claimed is:
 1. A base station apparatus that transmitscontrol information using a first control channel and a second controlchannel, the first control channel using a common reference signal thatis receivable by all terminal apparatuses, the second control channelusing the common reference signal or a specific reference signal foreach terminal apparatus, the base station apparatus comprising: a searchspace configuring section of a first integrated circuit configured toconfigure a search space having a plurality of allocation candidates, ina first resource region to which the first control channel is mapped anda second resource region to which the second control channel is mapped;and an allocating section of the first integrated circuit or a secondintegrated circuit configured to allocate the control information to aresource in one allocation candidate from among the plurality ofallocation candidates in the search space, wherein: a ratio of a numberof the allocation candidates in the second resource region to a numberof the allocation candidates in the first resource region in the searchspace configured in the terminal apparatus using the specific referencesignal in the second control channel is equal to or greater than a ratioof a number of the allocation candidates in the second resource regionto a number of the allocation candidates in the first resource region inthe search space configured in the terminal apparatus using the commonreference signal in the second control channel.
 2. The base stationapparatus according to claim 1, further comprising a generating sectionof the first integrated circuit, the second integrated circuit, or athird integrated circuit configured to generate the control informationin a first format or a second format, the first format being a formatcommon to all the terminals, the second format being a formatcorresponding to a transmission mode configured for a terminal, wherein:the allocating section is configured to allocate the control informationin the first format to the search space configured in the first resourceregion and allocates the control information in the second format to thesearch space configured in the second resource region.
 3. A terminalapparatus comprising: a first receiving section of a first integratedcircuit configured to blind-decode a search space having a plurality ofallocation candidates configured in a first resource region to which afirst control channel is mapped and a second resource region to which asecond control channel is mapped, the first control channel using acommon reference signal that is receivable by all terminal apparatuses,the second control channel using the common reference signal or aspecific reference signal for each terminal apparatus, and obtainscontrol information transmitted using the first control channel or thesecond control channel; and a second receiving section of the firstintegrated circuit or a second integrated circuit configured to receivea downlink data signal based on the control information for the terminalapparatus allocated to one of the plurality of allocation candidates,wherein: a ratio of a number of the allocation candidates in the secondresource region to a number of the allocation candidates in the firstresource region in the search space configured in the terminal apparatususing the specific reference signal in the second control channel isequal to or greater than a ratio of a number of the allocationcandidates in the second resource region to a number of the allocationcandidates in the first resource region in the search space configuredin the terminal apparatus using the common reference signal in thesecond control channel.
 4. A transmission method performed by a basestation apparatus that transmits control information using a firstcontrol channel and a second control channel, the first control channelusing a common reference signal that receivable by all terminalapparatuses, the second control channel using the common referencesignal or a specific reference signal for each terminal apparatus, thetransmission method comprising: configuring a search space having aplurality of allocation candidates, in a first resource region to whichthe first control channel is mapped and a second resource region towhich the second control channel is mapped; and allocating the controlinformation to a resource in one allocation candidate from among theplurality of allocation candidates in the search space, wherein: a ratioof a number of the allocation candidates in the second resource regionto a number of the allocation candidates in the first resource region inthe search space configured in the terminal apparatus using the specificreference signal in the second control channel is equal to or greaterthan a ratio of a number of the allocation candidates in the secondresource region to a number of the allocation candidates in the firstresource region in the search space configured in the terminal apparatususing the common reference signal in the second control channel.
 5. Areception method comprising: blind-decoding a search space having aplurality of allocation candidates configured in a first resource regionto which a first control channel is mapped and a second resource regionto which a second control channel is mapped, the first control channelusing a common reference signal that is receivable by all terminalapparatuses, the second control channel using the common referencesignal or a specific reference signal for each terminal apparatus, andobtaining control information transmitted using the first controlchannel or the second control channel; and receiving a downlink datasignal based on the control information for the terminal apparatusallocated to one of the plurality of allocation candidates, wherein: aratio of a number of the allocation candidates in the second resourceregion to a number of the allocation candidates in the first resourceregion in the search space configured in the terminal apparatus usingthe specific reference signal in the second control channel is equal toor greater than a ratio of a number of the allocation candidates in thesecond resource region to a number of the allocation candidates in thefirst resource region in the search space configured in the terminalapparatus using the common reference signal in the second controlchannel.